There is a saying that the heart of a fighter plane is its engine, and the soul is its pilot. This is a story of lost souls.

"Piece of Cake," told in six parts, begins tonight at 9 on PBS's Masterpiece Theatre. Airing near the 50th anniversary of the Battle of Britain and based on the brilliant black-comic novel of the same name by Derek Robinson, it is the saga of the fictional Hornet fighter squadron from the beginning of World War II in September 1939 through August 1940.

Host Alistair Cooke explains in his introduction that "a piece of cake" is slang popularized by the Royal Air Force in World War II. Its meaning connotes a task that lies somewhere between a cinch and a pleasure, and it reflects the squadron's attitude toward the war at its outset. But the phrase takes on a darker meaning as the war and the series go on.

This entertaining program should be of enormous interest to history buffs and to fans of Robinson's novel. Even the general viewer may be caught up in this story of men caught up in the early phases of the Second War.

The story opens at a time when the lines of battle are not clearly drawn. In those days, it made -- maybe -- as much sense to attack the Russians as it did to take on the Germans. It was a time when one of the challenges of a green fighter pilot was trying to tell the difference between his planes and the enemy's, a time when a pilot was as likely to get killed climbing out of his plane as he was hunting for Germans.

Though the men of the squadron are initially bored by the lack of combat, they eventually are all but overwhelmed by the German air force during the disastrous Battle for France, which preceded the more celebrated Battle of Britain. British pilots, who thought themselves the best in the world, are suddenly up against more experienced pilots using better tactics.

Just how disastrous the Battle for France was for the RAF can be measured in its lost Hurricane fighters. Of 261 sent to France, only 66 got back to Britain. And while most of us equate the Battle of Britain with the blitz and burning cities of London and Coventry, the first targets of Luftwaffe bombers were, in fact, RAF fighter bases. The German goal was to knock them out, thus gaining command of the air for an invasion. They came alarmingly close to pulling it off before Hitler switched the targets to British cities.

In the book, the pilots fly Hurricanes; in the film they fly the better known Spitfire.

Producer Andrew Holmes said that "there are, sadly, only two Hurricanes flying today in the world, and they hardly form a squadron. But fortunately, there are a dozen or so Spitfires."

The show uses five Spitfires (and several German craft), all flown by British airline pilots and led by Ray Hanna. Hanna is a former leader of the Red Arrows, Britain's famed military precision flying team.

Well, it pays off. Simple scenes of the Spitfires soaring in and out of the sky, coupled with a memorable film score, become well-nigh poetic. Stunts, such as flying under bridges, are brilliant. When all is said and done -- though purists may disagree -- Hurricanes and Spitfires were comparable craft in many ways, both powered by the legendary Merlin engine (as was the American P-51 Mustang).

But the series is about men, rather than machines. And it is fiction, not a documentary. A number of British critics complained that the series portrays not a portrait of gallant "knights of the air" but a portrait of complete misfits.

However, host Cooke noted that younger RAF pilots said they have known all the "Piece of Cake" types in RAF squadrons -- perhaps not all at once in one squadron, but at one time or another.

"Certainly pilots did fly under bridges for a lark" as one "Piece of Cake" pilot does, Cooke said. "Winston Churchill didn't say the men of the Royal Air Force were angels or even all fine human beings. He said 'They saved Britain.' Which they did."

Hornet squadron's philosophical adjutant, "Uncle" Kellaway, expertly played in the movie by David Horovitch, sums up fighter pilots in the novel. He explains to another officer, "They're all a bit mad you know ... They're not what you'd call model citizens, any of them. More like vandals, I suppose."

He also says that if you show the average pilot "something, anything, really, deep down inside, his first reaction is: 'What sort of mess could I make of that with a couple of three-second bursts?' Herd of cows, double-decker bus, garden party -- makes no difference what it is. I've often thought it's a damn good job they're in the RAF, otherwise they'd all be out blowing up banks."

As the series rolls on, war changes from a first glimmer of glory to tragedy and boredom, with an overlay of farce. The series cast changes gradually from episode to episode, as injury, death and madness overtake the young pilots.

In the first episode, some characters stand out. There is, of course, the adjutant, a veteran of the First War. Then there is Fanny Barton, played by Tom Burlinson, a sometimes-bewildered but well-meaning pilot confronted with unexpected responsibility.

"Skull" Skelton (Richard Hope), the skeptical squadron intelligence officer, does not really blend in with the pilots, many in their teens. He is a peacetime Cambridge don whose academic speciality is radical thought in England's northern counties during the Elizabethan Period. He has never been in an airplane.

The first pilot to make his mark is "Moggy" Cattermole, played by Neil Dudgeon, who may be the squadron's best pilot but is a disreputable human being.

Introduced near the end of the first episode is a new pilot, Christopher Hart (played by American actor Boyd Gaines), an American who flew for the Republicans in the Spanish Civil War. Hart is destined to clash mightily with the character who steals the first episode, Squadron Leader Rex (Tim Woodward).

Rex knows everyone in Britain worth knowing, gets the squadron officer's rations from exclusive London shops, has his car transported to France, and has been waiting for command and war all his life. When the squadron goes to France, he secures a banker's luxurious chateau as the squadron's new home, where the officers dress for dinner and dine with silver, crystal and china.

It is Rex who gives the book and series some of their most memorable lines. For instance, he is eloquent in briefing his men on what to expect from French flyers, drinkers and women.

"There are three things you should know about France," he says. "One is that the French are a nation of appalling alcoholics. If anyone offers you a drink, take it; you'll be doing him a favor. The second is that French women are staggeringly beautiful. When they ask you into bed, take your boots off first. If you enjoy it, take your hat off afterwards ...

"The third thing to know is the French suffer from the strange delusion that they invented flying. Some of them think they're Napoleon, too. They're all a bit loopy, in fact ... Everyone ready? Off we go."

A minor quibble with Cooke's otherwise excellent introduction to the first episode: While regaling us with the sad tale of the "phony war" in which the British and French armies and air forces did nothing to help Poland, he glosses over Britain's senior service, the Royal Navy, which did bloody battle with Germany from the war's first month. Nor was the campaign for Norway the walkover Cooke implies.

There may be some American viewers who wonder about the presence of American pilots in the RAF. They don't wonder about that in Britain.

According to Kenneth G. Wynn, in his book "Men of the Battle of Britain," four American RAF pilots were killed in the Battle of Britain in 1940. They were among 11 Americans, and a 12th who became a British citizen after going to England, who fought in the battle. Five American flyers who survived the battle were killed later in the war.

The four killed in the Battle of Britain were Carl R. Davis, William M.L. Fiske, Otto J. Peterson and Hugh W. Reilley.

Those who might think that the fictional career of Christopher Hart in "Piece of Cake" is too colorful to be believed would find it pale beside the life and death of Billy Fiske.

Fiske, the son of a banker, lived in France. He also had led the four-man U.S. bobsled team to victory at the 1928 and 1932 Winter Olympics. He joined the RAF in 1939 and downed a German bomber in August 1940. Later that month, while dueling another bomber, he was shot down and died a day later.

He was buried in a Sussex churchyard and is remembered in a plaque in St. Paul's Cathedral as "An American citizen who died that England might live."