Bob Grimes said he wanted to show me something. It was another piece of the puzzle, a key to the series of events he's been reliving for 60 years.

He wouldn't tell me much more than that as we maneuvered through the traffic westbound on U.S. Route 50 one recent afternoon. We were heading toward the new Smithsonian Air and Space Museum -- the Steven F. Udvar-Hazy Center at Dulles International Airport. As we drove, he tried to explain what it was like to be stuck on the events of a single afternoon that ended long before most people on the planet had been born.

"Well, I might be just looking out this windshield here," he said, raising his arm and sweeping his hand along the horizon, "or maybe watching a football game, and it'll come back to me, far as that goes. I know that it's just the way things were. But, in my mind, I'm back in the cockpit, left seat, looking at the controls, and I'm dodging and diving around the Nazi fighters, trying to make it to a cloud bank. And I look for every option, but I never come up with anything to save us. You start thinking about it again, and it's something that never goes away. You never stop thinking about it.

"And, you know, I'm positive I can still fly the thing. I can picture every control."

The thing was a B-17, the plane he piloted over Nazi-occupied Europe, and, on one mission in particular, over Belgium. That was on the afternoon of October 20, 1943. He's been trying to get his crippled bomber back to base ever since. It never works.

WE MET TWO YEARS AGO as I was researching a book about the Comet Line, a World War II European resistance organization that helped about 800 downed Allied airmen evade the Nazis and cross the Pyrenees Mountains from 1942 to 1944. Surviving Comet Line members kept a list of everyone they saved, and among them was a man named Lt. Robert Z. Grimes.

Bob is a retired Air Force colonel with 30 years in the service who has lived for the last 20 years in Fairfax County. His story became the narrative thread of my book, which describes his rescue by this group of brave young Belgians, French and Basques, who nursed his wounds and then moved him through hundreds of miles of enemy territory to Spain.

Bob is 81 and still robust, tall and handsome. He has a favorite picture of himself from the war years wearing his leather flight cap and flight jacket, looking up at the sky. Think Gary Cooper or Jimmy Stewart. He has a ready smile, a gray cowlick occasionally drooping over his forehead. He has a loving wife and family. In the years since he came home from the war, Bob and his wife, Mary Helen, raised three daughters and now have two grandsons. It's been a good life with few regrets. In retirement, he exercises every day, including lifting weights and doing calisthenics; goes for walks; plays regular rounds of golf with friends; gardens; reads; and surfs the Internet. His major complaint is that his years in the Air Force, decades on the flight line before noise protection devices were in vogue, have left him hard of hearing.

He attends yearly reunions of the Air Forces Escape and Evasion Society, veterans of World War II, and visits with friends who are retired officers. Bob is not one to brag about his life, nor call attention to himself. He says he sees no reason to play up "the military thing." But Bob enjoyed our interviews, summoning forgotten aspects about his time in occupied Europe. It helped him work out more of the puzzle. The details made it evident that he was extremely lucky. While his rescuers shielded him in Brussels, Paris and southern France, the Nazis stepped up efforts to break the resistance and destroy the escape lines. Some of his friends were imprisoned; others never made it home.

BOB GRIMES JOINED the Army Air Forces in 1942 and bounced from one training course to another, learning how to fly biplanes, then two-engine jobs, slowly mastering the basics.

He arrived in Blytheville, Ark., in the late fall of 1942 for cadet training. Blytheville is a town of less than 20,000 in the Mississippi Delta. What he remembers most about his six weeks there was meeting a pretty sorority girl home for the holidays from the University of Arkansas at Fayetteville. Her name was Mary Helen Moore. He met her on a blind date set up by friends at a Christmas party dance. There was Bob, standing tall in his cadet uniform, and there was Mary Helen, a lovely young woman with brunette curls, walking down the stairs to greet him.

Mary Helen didn't need to look any further. The question of choosing a mate was settled, even though she was 19 and Bob had just turned 20. "I saw him, and I knew," said Mary Helen, seated at their kitchen table in Fairfax. "We started seeing each other as often as we could after that." Stealing time together required a little bending of the rules, facilitated by Mary Helen's father, L.H. Moore, a local dentist. Bob suddenly found the urge to have frequent dental work. "That would get me off the base, which wasn't easy," Bob said. "I actually did get down to his office once in a while, but it was only to have him sign my pass for being off base."

Bob completed his training on B-17 bombers, known as "Flying Fortresses," and prepared to ship out in the summer of 1943. But before he went, he called Mary Helen and hinted they'd have some plans to make when he came back from the war.

Bob and his nine-member crew picked up a brand-new B-17 at the Spokane Air Depot and headed for Europe, where they were assigned to the 96th Bomb Group based at Snetterton Heath airfield, about 100 miles northeast of London.

All year long, British and American bombers had been flying increasing numbers of sorties over Germany and the occupied mainland. The U.S. planes flew the daytime missions, and the British flew at night. That left the Americans far more vulnerable, and the Nazis had been successful using antiaircraft guns and swarms of fighters to devastating effect. Bob's first combat flight in September 1943 was a piece of cake. He noticed some groundfire, but it wasn't too bad. The second mission was over Frankfurt on October 4, a week later. Same routine: Take off at dawn, make formation and fly wingtip-to-wingtip to the target. Frankfurt was no problem; Bob did notice more groundfire and fighters, but he flew without incident. The third mission came four days later, a bombing run on the port city of Bremen. This time, German ground-based antiaircraft fire saturated the sky. A cloud of flak was suspended in the air ahead of them. There was nothing to do but fly right into it. Still, it all went according to plan; the planes dropped their bombs on the target and went home.

It was a rare occurrence to have good weather on two consecutive days, but October 9 was forecast clear, and the U.S. 8th Army Air Force Command took advantage. The mission was a long-range flight to Gdenia, Poland, now Gdansk. This time, Bob and his mates dropped their bombs under a heavy assault, and at least six B-17s out of 109 making the run were shot down. Because of fuel limitations, the return trip was a straight shot at 28,000 feet over Germany. Along the way, they hit flak once more. At 170 mph, the razor shards of metal looked like a black cloud flecked with dust and sounded like rocks or baseballs against the side of a house. As they were flying along through the flak and the smoke, the plane suddenly buffeted.

A shell had burst right in front of Jerry Nawracaj, the waist gunner, who sat at a side window about midway between the wings and the tail, from which he pivoted and fired his 50-caliber machine gun. Suddenly, Nawracaj was flat out on the deck with the machine gun in his lap. Through some miracle, he was fine.

After some tense moments, the groundfire trailed off, and Bob was over the English Channel. He remembered a smooth, uneventful landing. On the ground, the crew piled their gear onto a truck for the ride back to base.

But Bob held back for a minute and walked around the airplane. There was a gaping hole where the gunner's window used to be. The rest of the fuselage was shot up like Swiss cheese. It was just blind luck that the wiring and engines and fuel lines were intact. He had a sick feeling about all the damage. After debriefing, Bob went to look for the chaplain, just to talk, even though he didn't know what he wanted to say. The chaplain wasn't around, though, as Bob recalled. He was at the officer's club getting drunk.

AN AIRBUS GLIDED OVERHEAD toward landing as Bob was talking; we were close to the museum now, an enormous hangar located just off the end of a runway at Dulles. We looked up at the plane, a mass of modern engineering, computers, telemetry.

Bob recalled his training in a PT-17 biplane, whose instrumentation consisted of an on-off switch, a throttle and a stick, with a level to maintain the horizon. Recently, he attended a veterans reunion in Wichita Falls, Tex., and was introduced to a group of swaggering top guns at Sheppard Air Force Base. Bob told them how he flew his B-17 in the dark, no lights, no radar, with lots of other aircraft all over the countryside, occasionally signaling to one another with flares. He said the modern-day pilots were frightened thinking about it.

Bob had been to the museum on opening day last fall as an invited guest, along with Mary Helen. "Ten thousand of our closest friends," Mary Helen had joked. It was too crowded that day to stroll around at leisure, even though the museum covers 760,057 square feet over 176.5 acres. This weekday might be better. "And I have something I really want to show and explain to you about airplanes," he said again. "This will really explain a lot."

Walking into the hangar through the arched entrance, we saw the titanium black carcass of a Lockheed SR-71A Blackbird and to the left, the graceful lines of an Air France Concorde. The immediate impression was distortion of scale. The building was so large that the massive airplanes appeared reduced in size.

Bob quickly headed down the ramp to the main level, where he turned away from the airplanes to a small corner of the room, hidden from view. "Let me see if I can find what I'm looking for," he said, stopping and starting several times. Finally, we stood before a pane of glass containing a fluted piece of metal about the size of a canned ham. "This is it, a supercharger, very similar to the one on my B-17 that day." This is the culprit.

The supercharger on a B-17 was a relatively new invention in 1943, a device that forced compressed air and exhaust back to the engine to produce more power.

"When it works, everything is fine. When it doesn't work, you can't tell . . . when you're on the ground. But when you hit 10,000 feet, and the air pressure starts to drop, if it's malfunctioning, you start to notice, and it becomes a drag on the engine. That's when it's too late."

That's what happened on Bob's fifth mission -- October 20, 1943. Because of the damage his last time out, he was given a replacement aircraft with the nickname Shack Rabbit II. Bob and his co-pilot, Art Pickett, went through the preflight checklist. Bob ran up all four engines, and they sounded fine. It was late morning when he taxied the plane and his nine crew members to the end of the strip. Bob adjusted his oxygen mask, moved his hands to the controls, steadied his feet on the rudders, took a look at the pressure gauges and double-checked the fuel readings. There was a deep growl as the plane creaked forward until he let out the brakes, and Shack Rabbit II lumbered down the strip. It took every bit of runway to lift the 26 tons of metal off the ground at about 110 mph, practically skimming the treetops on its slow climb.

Bob remembers a looping ascent over England to meet up with his squadron past the mouth of the Thames River along the southeastern English shoreline. The planes drew together one by one, creating a six- or seven-plane group, sometimes only a wingspan apart. He was constantly fighting turbulence and prop wash from the other planes, adjusting the speed, trimming the tail and leveling the wings.

He kept the climb steady over the English Channel, put on his gloves as the temperature went down and, before long, saw the European coast in the distance. Just about then, he noticed that the needle on the manifold pressure gauge on engine No. 4 was dropping, indicating the engine's supercharger was failing. At first, he tried to ignore it, but the higher the squadron went, the lower the pressure. As the engine lost power, the manifold pressure decreased, and the propeller turned more slowly. Instead of adding to the plane's thrust, the propeller began to act as an airbrake. The rest of the planes were still climbing, but Bob started falling back. Someone else moved into his position on the right flank. Finally, it got so bad that Bob had to cut power on that engine and feather it to reduce the drag. He pushed the throttle on the remaining three props to get more power, fighting back to the formation. But as they crossed the Belgian coast, the planes hit a wall of bad weather. The lead plane lifted its nose to climb again above the top of the clouds.

But Bob's plane couldn't climb any higher. He was stuck down below; the easy prey that enemy fighter pilots liked to dine on. Within minutes, at least six German FW-190s were coming at Shack Rabbit II from all directions. Head-on, a fighter was visible at the windshield, its guns resembling blinking light signals as it fired directly at him.

"Bandit, bandit," the men called on the intercom as they pivoted about, trying to hit the attackers with a spray of fire from their 50-caliber guns.

The enemy's machine guns were blazing, and the plane buffeted as Bob aimed for a low patch of clouds to hide in. The plodding B-17 could not outmaneuver planes flying twice as fast. Seconds more, and there was a jolt, then a clanking noise like a rattling sardine can.

A 13mm machine gun shell hit the inside cowling of the No. 2 engine next to the pilot seat, broke in half and ricocheted end-over-end through the fuselage. It cut through Bob's flight suit, sliced into his left quadriceps, penetrated the soft tissue and lodged along the upper quadrant of his femur. He says it felt like someone had hit him in the leg with a hammer.

Bob shook his head clear and saw another engine was smoking, hit by the sparking flashes from the FW-190's 20mm cannons. He felt more shudders on the stick, and then the plane refused to respond to the rudder. He pulled back to keep the nose up and feathered the smoking engine.

Theoretically, the plane could fly on two engines, but Bob remembers making a quick calculation. He could hardly keep flying straight; the two remaining engines were yanking the right wing down. He had no choice: He hit the buzzer to abandon ship and started counting slowly to 120, the number of seconds allotted in their training for strapping on their parachutes and jumping from the airplane. But he knew it wasn't going to be easy to keep the plane flying that long.

Pickett, the co-pilot, was first to go. Stripping off his mask, he reached for the parachute under his seat and clipped it to his front harness. He went down through the hatch to the middle deck and jumped from the side door. Bob, meanwhile, played for time. He remembers counting down the seconds and struggling to hold the plane steady. Time was running out fast. He looked up at Ted Kellers, the engineer, who was still firing the machine gun in the turret above him. He should have bailed already. It was typical of Kellers -- he wanted to keep fighting.

Get out, get out! Bob yelled.

Seconds later, Bob decided there had been enough time for the rest of the crew. He doesn't remember squeezing down off the flight deck; he doesn't remember the wind blowing through the hatch in the fuselage. He saw Kellers thrown against the floor, fumbling to get his parachute clipped on right. Then Kellers jumped into the sun.

Bob doesn't remember jumping. At one instant, he could see the controls and hear the sound of metal popping and the constant whir of the engines. Then he was sucked into the sky in a torrent of wind.

It was calm and quiet, and he was plunging through the air, mouthing words that were taken by the wind, "Heaven, this must be Heaven."

Bob learned after he got back to the States that Pickett, his co-pilot, was killed falling to the ground when his chute malfunctioned. Three other men died in the plane, shot up in the air battle. Nawracaj, the right waist gunner, didn't make it through. Frederick MacManus, the radio operator, and George C. Janser, the tail gunner, also died. The other five members of Bob's crew survived, including Robert Metlen, the ball turret gunner. That was a comfort: Metlen would have had the most trouble getting out of the plane. That meant Bob could conclude that he had kept it flying long enough. It later became clear that the B-17's tail had been shot away and there would have been no chance of regaining control.

But that's not good enough for Bob. At the museum, he was thinking about the four dead friends he was responsible for. "You brought the crew together, and you trained with them; you really didn't want to talk about it, when you had four missing crew members," he said. "When you talked about it, you tended to leave out the bad parts."

But when you think about it, the bad parts won't go away. They're what keeps Bob in the pilot's seat trying to figure out how it could have been different. One option was to turn around at the first sign of trouble with the supercharger and return to base. But that would have gone against his training and his prime directive -- get your bombs to the target.

He and his fellow pilots were under pressure from the air command. Former defense secretary Robert McNamara discussed that pressure in the recent film about his life, "The Fog of War." McNamara was a statistician at the time on the staff of the 8th Air Force, which was Bob's outfit. In 1943, the command was trying to hit the Germans with as much firepower as possible. Twenty percent of the missions were scrubbed by pilots who came home with nothing apparently wrong with their planes. McNamara's conclusion -- they were scared. Pilots were warned to complete their missions.

Bob was not scared: "I thought I could do it until the weather showed up. And after that, it was too late. What would have happened if I returned to base with my bombs, and the supercharger checked out fine at normal ground pressure? What would I have said?"

BOB AND MARY HELEN INVITED their three grown daughters over for a Saturday lunch. Sitting at the dining room table, they ate finger sandwiches of egg salad and tuna, and spoke lovingly. It became clear that even now, they didn't know exactly what had happened to Bob in occupied Europe. The military stamped "TOP SECRET" on everything to do with the experience of crashing in Europe and evading capture. No one ever told Bob when it was all right to start talking. But that's not the whole reason why he never spoke much of that time.

He and Mary Helen figure that most veterans were the same way, trying to bury the painful parts as they looked ahead to careers and raising families. Things changed when retirement came -- people started looking for lost friends, and modern technology made that easier.

The daughters said they knew only the general outline of their father's ordeal during the war. Dale Soper, who is a schoolteacher in Baltimore, is the oldest, born in Europe after the war. She and her sisters considered themselves Army brats, with a hero for a dad. "I thought my dad was this tall, dashing hero, the bravest man in the world," Dale said, "but we never knew any of the details."

Susan Grimes, the middle daughter, said they all knew that Bob was special. "But there were other fathers going around who also had medals. I have a clear memory of it being something that you don't ask, that he wasn't too comfortable talking about."

Jennifer Klostermeyer, the youngest, said that much of the mystique surrounded the bullet fragment that Bob had preserved all this time. "One of my lasting memories was seeing the bullet holes from both sides of his leg," she said. "And then, he had the bullet. I'd sneak up to the night table stand and look at the bullet."

What about the bullet? The girls agreed it was an icon of their dad's heroism and experiences, but on a winter day more than 60 years after the fact, they looked back with blank expressions, and shook their heads. They still didn't know the story.

THE PARACHUTE BROUGHT BOB DOWN near a road in rural Belgium. He hid until nightfall, vaguely aware of his bloody, wounded leg. He heard dogs, and he assumed that German soldiers had seen his parachute and would be out searching for him. He was lying in a small thicket of trees that gave him shelter, and when night fell, he decided to look for help. People shooed him away at first, but he begged for aid and showed the blood on his uniform. He was taken in by a farm family and, through their contacts, reached the members of the underground Comet Line.

Bob was shifted quietly at night from house to house, always one step ahead of the Germans in hot pursuit and with his leg wound making it almost impossible to walk. After two days, his leg was swollen and Bob fell into fever and delirium. A nurse came up with sulfa powder and pills, which brought down the infection.

When Bob was able to walk again, resistance members smuggled him to Brussels. They hid him in a safe house with two women, one of them a young nurse, code-named Lily, who arranged for a doctor brave enough to take care of him. Harboring an escaped airman meant death. There were informers everywhere. Emergency surgery using anesthetic and sutures would attract unwanted attention.

The doctor and Lily drove Bob to a clinic, pretending that he had a sore throat. As they prepared for surgery on his leg, they told him they couldn't use any anesthetic. Also, he'd have to leave the clinic quickly, walking under his own power.

Using a fluoroscope, the doctor located the piece of shrapnel, a fragment of a shell, lodged at the femur. He gave Bob a towel to clamp down on with his teeth. Bob bit down hard on the towel and tensed his arms. He felt like he was in a John Wayne cowboy movie. The doctor made a mark on the inside of Bob's left leg, on the opposite side from where the bullet entered. Just as the cutting was becoming unbearably painful, the doctor reached in with a small set of forceps, plucked out the bullet and dropped it into a pan.

Bob carried the bullet in his pocket for six weeks while he regained strength in Brussels, then across France, through Paris by train and then by bicycle to the foothills of the Pyrenees. The Comet Line led him over the mountains on Christmas Eve 1943. Wading across a flooded river, he reached safety in Spain, was shuttled to Gibraltar and back to England.

All the while, Mary Helen was in touch with Bob's mother by phone. "They said that he was shot down, that he was listed as missing in action, and we didn't know any more than that," Mary Helen said. "By December, I thought, 'Well, it's too long, and he might not have made it.' " She was trying to be detached as she told this story, but her eyes were welling up. The girls at her sorority were hearing lots of cases of boys they knew who weren't coming home. She said there was a resignation to it, along with hope. What more could they do?

"Finally, there was a telegram from England through Washington. And we knew he was coming home."

Bob and Mary Helen were married in Blytheville on August 6, 1945. It was the day the United States dropped the first atomic bomb, on Hiroshima. News of the bomb didn't come out until the next day. Until then, Bob had been training pilots at an air base in Apalachicola, Fla., and had been called for a strategic planning course at Fort Leavenworth, Kan. "I thought I was going to be involved in the invasion of Japan," Bob said. Less than a month after he and Mary Helen were married, the war was over. Bob kept flying and became an officer in the newly constituted U.S. Air Force.

He flew the Berlin Airlift of 1948 and 1949, served in France, rose to the rank of colonel and was on the general staff at the Pentagon when he retired, a 30-year-man, in 1972. After that, he was an assistant superintendent of schools in Prince William County, where he attended to logistical details with the dedication and thoroughness he had shown as a pilot. He could describe with all due seriousness the importance of deciding whether to cancel school on a winter day when conditions might be fine in one part of the county but snowy and icy elsewhere. Details matter.

The girls grew up on military bases in France, Germany and the United States, before the family moved to suburban Washington in the 1960s. Bob and Mary Helen live in a pleasant split-level house not far from George Mason University. In the old days, many of the neighbors were also military types, Mary Helen said. "We tended to see them and stick close together."

But there was little conversation about wartime experiences until Bob and the others reached retirement age. Talking about World War II came into vogue, and the men who had spent time with careers and raising families started finding one another and setting up reunions.

For Bob, the desire to relive the experience didn't go away. Then, in 1993, he was invited to attend a 50th anniversary celebration in Belgium in the town where his plane went down. He didn't know what to expect, so, in his organized way, he created biographical sheets on each of his crewmen, along with the assignments and where they went down. It gave him a reason to contact the family members of those who had died. But he found that their relatives preferred to leave the past alone.

He called the number of one crewmate's brother. A woman answered, and there was a pause on the line when he said who he was. After a while, she came back on.

"She said, 'No, no, he can't come to the phone, he doesn't want to talk.' "

"When I told them I was going back to Europe in 1993, they didn't want to hear about it. That's when it really comes home to you. I wasn't soliciting anything from them. But we all wished it could have been different."

It left Bob thinking about two minutes on an airplane, flying for the clouds, and the split-second decisions made by a 20-year-old pilot with his life ahead of him and a war to fight. He was humbled by his fortune but could not help comparing it with that of the men he lost on October 20, 1943. That was what brought him back to the cockpit.

The people in Belgium gave Bob some comforting perspective. He was a returning hero, and everyone in the town remembered the night that a B-17 came down and the Nazis were hunting for a young pilot with a torn leg. There was a parade for Bob. They named the town square after Art Pickett, whose parachute failed to fully open. And the bodies of his crewmen were reburied together at Netherlands American Cemetery and Memorial in Margraten. He remembers standing there in the sun, people walking up to him, tears in their eyes, 50 years after their liberation from the Nazis. One man gave him a piece of shiny metal that had been hammered into a shoehorn. It was a piece of the fuselage of Shack Rabbit II. Someone else gave him a lace collar that had been fashioned from Bob's parachute.

Bob Grimes may never remake October 20, 1943, into the neat, orderly day that his mind and his spirit would like it to have been. And when he hears people talk about heroes, he says that he was part of the larger picture, doing a job that had to be done.

He just wishes it could have been different.

Peter Eisner is a deputy foreign editor at The Post. This article is adapted from his book The Freedom Line, The Brave Men and Women Who Rescued Allied Airmen From the Nazis During World War II, recently published by William Morrow.