On Aug. 1, Vitaly Yurchenko slipped into the U.S. Embassy in Rome with one goal: he wanted a new life. Frustrated by a stagnant spy career in Moscow, despairing over failed relationships with his wife and son, lovesick for a Russian mistress in Canada, he would "disappear," and secretly defect to the country most eager to buy what he had to sell.

Or: On Aug. 1, Vitaly Yurchenko slipped into the U.S. Embassy in Rome to set off the most dazzling "sting" operation in modern espionage history. Reeling from recent defections of key Soviet agents to the West, infuriated by American gloating, desperate for an intelligence coup of its own, the Soviet government would plant Yurchenko as bait in a trap timed to snap shut before a worldwide audience, turning U.S. exhilaration to embarrassment.

Which version is true? In spy stories of this kind, truth is a rare commodity. An intelligence analyst would ask, which version is most credible? A layman might reply that Yurchenko's three-month odyssey through the U.S. intelligence apparatus and out of it again was totally incredible, even mind-boggling.

"You could sit two people down with exactly the same set of facts, and they would come up with opposite conclusions: He was a double agent; no, he was a defector who became depressed," said Sen. David L. Boren (D-Okla.), a member of the Select Committee on Intelligence. "I can argue it round or I can argue it flat. It comes down to your own intuition." Even the consequences are unclear. Some U.S. officials say Yurchenko spilled valued information. Others say he gave and took little. Maybe he was just a bird that flew into the jet engine: a freak accident, not a systemic failure.

At the least, he escorted a wide-eyed world into the secret and seductive realm of espionage, where the bizarre and unexpected are commonplace, where the most skilled practitioner is prone to see the implausible as plausible, the unreal as real. In this realm, illogic harbors a logic all its own.

The tale is beyond logic, from Yurchenko's disappearance in Rome to his tete-a-tete over supper with America's chief spymaster in Langley, Va., to an unprecedented, internationally televised news conference in Washington, where he said he was drugged and kidnaped?

Here is the tale, told twice, of V.S. Yurchenko: once as genuine defector, once as double agent. The facts are the same, only the motivation -- and therefore the interpretation -- changes. The motivations offered here were suggested by intelligence officials and experts, but they are largely guesswork.

In time, more facts may emerge to make one version markedly more persuasive, or that introduce a new explanation, such as the intriguing scenario that Yurchenko was deliberately sent back to Moscow by the Central Intelligence Agency as a triple agent. The Genuine Defector

Yurchenko, 49, was entering middle age, unhappy in love and in work. His heart belonged to a woman in Ottawa, the wife of a Soviet Embassy official whom he had known since the 1970s.

According to the CIA, he was listed as No. 2 in the KGB department overseeing spying in North America, but his career was arguably going nowhere. He was the security officer, an inglorious watchdog.

It was not the kind of life he might have envisioned in the late 1970s, when he was the feared chief of security at the Soviet Embassy in Washington and a sort of man-about-town known as "Vity" to bartenders in posh downtown hotels.

If he was looking for a way out, the Soviets unknowingly gave him one last July. They sent him to Rome to investigate the disappearance of nuclear scientist Vladimir Alexandrov, last seen in Madrid in April.

While strolling with Soviet officials on the hot summer morning of July 28 (according to Italian press accounts), he excused himself near the ambassador's residence, saying he wanted to tour the Vatican museum on nearby Via delle Fornaci.

He never came back.

On Aug. 1, as he walked into the U.S. Embassy, Yurchenko had reason to believe he was on the threshold of a new life, with plenty of money and security, and perhaps his sweetheart to boot.

And he was counting on his new protectors at the CIA, as fellow espionage types, to keep it all secret so that his superiors, his wife and most importantly his 16-year-old son would not know that he had abandoned the Soviet motherland. The evidence would show that he had "disappeared," a plausible end for a KGB agent.

Soviet Embassy officials reported Yurchenko missing on Aug. 2, providing Italian police with a description, but no photograph, and saying they suspected the CIA had kidnaped him. By September, Soviet officials here queried the United States on Yurchenko's whereabouts.

Meanwhile, U.S. officials whisked their prize defector to Washington under Attorney General Edwin Meese III's "parole power" for emergency immigrations, and installed him in a "safe house" on 10 acres about 90 minutes south of Washington.

Then, the debriefings began, and the CIA quickly found that Yurchenko was a good catch.

He exposed Edward Lee Howard, a former CIA agent fired from the agency who then betrayed to the Soviets a valued U.S. source in Moscow. He told them about three other Americans who had worked for Soviet intelligence, some sources say. He cleared up a decade-old mystery by revealing that the Soviets had accidentally killed U.S. double agent Nicholas G. Shadrin in 1975.

But the debriefings became emotionally depleting and ultimately depressing -- normal feelings, according to other defectors. Day after day, professional questioners picked Yurchenko's brain. He later complained that he had to speak English all the time, an added strain.

At night, he said later, he had to sleep with his door open under the watchful eye of a "fat, quiet, stupid -- excuse me -- unemotional person who is following the orders." During the day, he claimed, he was watched much of the time by six CIA officers, including a Vietnam veteran named Colin Thompson, who called himself "Charlie."

Within weeks, his new life in the United States began to look worse than the one left behind. Both Soviet and American sources agree that Yurchenko, while in U.S. custody, was treated for an ulcer -- "All of us have stomach trouble over here," a Soviet Embassy official said -- and medical authorities say the most commonly prescribed ulcer medication could have caused the defector to feel disoriented and confused.

In late September, with the help of the Royal Canadian Mounted Police, the CIA reunited Yurchenko in Ottawa with his beloved. But the woman spurned him, and Canadian sources observed him to be "emotionally upset" after the visit.

Meanwhile, far from keeping his defection a secret, U.S. officials crowed about him. Somehow, his role in the Shadrin and Howard cases leaked to the news media. He had a private dinner with CIA Director William J. Casey, and Yurchenko saw it reported in a Newsweek magazine article calling Yurchenko "the highest-ranking Soviet defector in years" and quoting a "senior administration aide" boasting, "He's for real."

On Oct. 11, after newspapers and television networks had been flooded with news of the Yurchenko defection, the State Department announced it officially.

Debriefers began trying to negotiate his life contract -- $1 million down and $62,500 a year for life in return for information, according to Yurchenko -- but he put them off.

Heartsick, lonely for someone to whom he could open his soul -- in Russian -- Yurchenko's thoughts may have turned to his adopted son, who had difficulty in school and was a discipline problem.

He knew that some defectors had been allowed home after declaring they were kidnaped and drugged, then swearing unswerving loyalty to the motherland. Maybe he, too, could pull it off.

The debriefers by mid-October had relaxed his security, emphasizing that he was free to go anywhere. On Saturday, Nov. 2, Yurchenko was assigned a fairly new employe, a young and inexperienced agent who agreed to take him to Au Pied de Cochon, a Georgetown brasserie, 1 1/2 miles from the Soviet compound on Tunlaw Road NW.

As they finished their dinner about 8 p.m., Yurchenko looked at the young agent, said he wanted to take a walk, and asked, "If I walk away, will you shoot me?"

Of course not, came the answer. This is America.

Perhaps out of fatherly feeling toward a young agent, perhaps out of anger at his debriefers, he parted with these words: "If I don't come back, don't blame yourself."

And with that, Yurchenko vanished.

He made his way through the big iron gates of the Soviet compound, apparently recognized by old friends from his days in Washington. According to intelligence sources, embassy officials likely consulted senior authorities in Moscow, possibly even the Politburo, to decide how to treat this traitorous defector.

It was Yurchenko who proposed a news conference, according to a Soviet official. It seemed a clever way to discredit the United States. The Soviets initially planned to hold it on Sunday, but realized that football games frequently preempt Sunday network news shows and that the publicity would be better on Monday, the Soviet official added.

The turncoat Yurchenko surely could be trusted with this unprecedented assignment because as an old KGB hand, he knew his life was riding on it.

And so, Yurchenko played his part like a master last Monday afternoon, accusing the United States of "state terrorism." But there was a tantalizing clue to his real thinking. When a reporter asked how he escaped, Yurchenko answered by talking of his son, convincing some skilled observers he was going home for the boy:

"I have a 16-year-old son and he had his problems with his studies and with his behavior . . . . I used to tell him that there can be no situation without an exit. If you really think about it you can always find a way out of any situation." The Double Agent

Soviet intelligence officials were desperate. Both the London station chief of the KGB and the deputy director of Soviet intelligence in Greece had defected to the West. Americans had boasted that it was like losing two CIA station chiefs to Moscow.

With the summit near, the Soviets urgently wanted to embarrass the United States, shake CIA confidence in their newly won defectors and stanch any future westward flow of intelligence.

They concocted a plot of rare audacity: plant a defector in the United States, manipulate the Americans into believing he is one of the KGB's top five officials, give him a few choice intelligence tidbits to divulge to enhance his cover, but mainly pump him full of disinformation.

Once the Americans take the bait, he escapes in a spectacular surprise ending, staging an unheard-of Washington news conference, leaving even some of the most skilled observers believing he was a real defector who "redefected."

And who better for the job than Vitaly Yurchenko? He was as plausible a defector as any -- competent in English, with an un-Russian, man-about-town air. He would tell his CIA handlers that he had tired of Soviet life and of an unsatisfactory marriage, and that he had liked the freedoms of America.

To sweeten the story, the Soviets would fabricate for him a love affair in Ottawa with an embassy wife. Intelligence types always believed that love, more than ideology, moved people to defect.

Yurchenko was also as tough as they came. He struck terror into embassy employes here as security chief from 1975 to 1979 -- a watchdog against defectors. And in any case, he did not have the kind of job that gave him access to the sort of information the Soviets most feared losing.

So off he went to Rome, into the U.S. Embassy.

Everything went as planned. The United States agreed to Yurchenko's basic demands for a new life, a reunion with his sweetheart in Ottawa and utmost secrecy -- with the Soviets knowing full well that in the open U.S. system, the news would leak.

As arranged, Yurchenko turned in Howard, who was believed to be of no more use to the Soviets. Howard escaped, apparently thanks to U.S. bungling. And Yurchenko explained Shadrin's mysterious 1975 disappearance, but that was only a footnote to history, anyway.

Still, U.S. officials trumpeted these as major intelligence coups, and in return, intelligence officers took Yurchenko in late September to Ottawa to see his lover. She spurned him as planned, possibly receiving in the process a valuable report on his first weeks with the Americans that she passed on to Moscow.

Meanwhile, the leaks in the media went beyond even the Soviets' wildest imaginings. They surfaced in so many different places, billing Yurchenko as such a big "catch" -- all on the authority of unnamed intelligence sources -- that the ombudsman of The Washington Post publicly chided the newspaper for failing to report Yurchenko's defection as prominently as its competitors.

Then Yurchenko hit the ultimate propaganda jackpot: an intimate meeting with CIA Director Casey in his private dining room. Before long, this meeting also was leaked by gloating intelligence sources, setting the Reagan administration up for a bruising fall: America's chief spy dining intimately with a Soviet plant.

Everything was in place for the grand finale. Knowing the CIA ultimately wanted to let him exercise his new freedom, Yurchenko persuaded his young handler to drive him into Georgetown, on the drizzly night of Nov. 2. At a noisy all-night bistro called Au Pied de Cochon, where one can easily get lost amid the crowds and the eclectic, Franco-Washingtonian decor, he excused himself for a walk.

He was welcomed back to the Soviet compound as a hero, and preparations were made for Monday's news conference in which he would charge that he had been drugged and kidnaped in Rome, then held prisoner. Every major news organization in the country by now knew Yurchenko's name and would give the session top billing.

Yurchenko, the star witness, appeared to have nothing to fear. This was hardly a defector, terrorized as to whether he would be executed or spared. This man was so composed that he brushed aside the third-ranking official in the embassy who sought to end the briefing before Yurchenko was through talking.

"Don't try to press me. I am used to such pressing . . . . It doesn't work against me," Yurchenko told a reporter who questioned his story. Then he peremptorily called an end to the questioning.

"Thank you for your time," he said smiling. "Bye-bye."