They were the Vietnam War era's only convicted spies, caught stealing U.S. documents for the Communists in a classic espionage case that had intrigue, romance, code names and surveillance personally approved by the president.

In the best modern spy thriller tradition, it also was a case laced with ambiguities. And Operation "Magic Dragon," as the FBI called it, had no heroes.

Yesterday, after 3 1/2 years of fruitless appeals, the two men--David Truong, onetime antiwar activist and Stanford-educated intellectual, and Ronald L. Humphrey, a former United States Information Agency employe--played out the final scene in Alexandria, the site of their highly publicized trial in 1978.

Rebuffed earlier this month by the Supreme Court, which refused to consider their claims of unconstitutional government behavior, Humphrey and Truong surrendered to U.S. marshals and were driven away separately in handcuffs and leg shackles to begin serving 15-year prison terms.

Neither spoke to or looked at the other. Both men said in recent interviews they have not communicated since the trial, although they lived within a few miles of each other, Truong near Dupont Circle and Humphrey in Arlington.

"I felt he deceived me," says Humphrey, a balding, 46-year-old ex-bureaucrat who was a USIA communications watch officer trying desperately to free the woman he loved from Communist-run postwar Vietnam when he first met Truong in Washington.

It was the bright, sophisticated Truong, many involved in the case agree, who played the major role in the relationship between the two men. "I don't believe we ever tapped Humphrey's phone because Truong was the spy and Humphrey was the feeder," says former U.S. Attorney William B. Cummings, who led the prosecution.

"I think that I was a very vulnerable target and they the Justice Department just decided to go after me," says Truong, the 36-year-old son of a man who ran for president of Vietnam in 1967 on a peace ticket. "I don't hold anything against Humphrey. He does what he thinks is important in his life and I do mine."

Truong was forwarding books, pamphlets and, most importantly, Humphrey's USIA documents by courier to Vietnamese associates in Paris at the time of Vietnam-U.S. talks on normalizing postwar relations when the FBI arrested both men. The courier, it turned out, was a CIA agent known by the code name "Keyseat."

The arrests followed the first foreign intelligence security investigation in which a president--Jimmy Carter--approved warrantless electronic surveillance including video tape cameras hidden in the ceilings at USIA and wiretaps and a microphone planted at Truong's apartment.

Truong, who maintained he trafficked only in information commonly known in foreign policy circles and frequently leaked by Washington officials, used his surrender for a news conference on the courthouse steps.

"At a time when our constitutional rights are being violated and eroded by this administration, I firmly believe that the injustice being done to me will be heard now more loudly and clearly, here and abroad, than ever before," Truong said as his American wife and about 30 supporters looked on.

As he spoke, Humphrey, dressed in a frayed red jacket and carrying an overnight bag, slipped unnoticed by most reporters through a side door with his Vietnamese wife, Kim, the woman whose plight in Communist Vietnam was an important element in her future husband's involvement in the conspiracy.

Humphrey met her during a tour as a USIA adviser in Vietnam from 1969 to 1971 and has said he believes she saved his life by warning him of a Vietcong death plot. "She was the girl next door," he says. When Saigon fell in April 1975, Kim was left behind and Humphrey, then in Washington, began frantic efforts to get her out.

During a visit to the Vietnamese-American Reconciliation Center, a Truong-created interest group in the District of Columbia, Humphrey made Truong's acquaintance. Soon he was visiting Truong's apartment at 2000 F St. NW and bringing copies of diplomatic cable traffic--much of it classified confidential or secret--with him.

"It wasn't long after he began dealing with Truong that Kim came out," says Cummings. "We made a big thing about that at the trial ."

Humphrey now says he believes that Truong's influence in freeing her was minimal. "It turned out he never did anything," he says. Humphrey says Swedish and West German diplomats and the International Red Cross assisted Kim's release.

In the meantime, a Vietnamese woman named Dung Krall, CIA code name "Keyseat," had infiltrated the Truong-Humphrey connection. Acting as Truong's courier, prosecutors say Krall was given a letter for Truong early in 1977 by Vietnamese close to their country's negotiators in Paris.

With President Carter's personal approval, the letter was opened by U.S. counterintelligence officers. On the strength of its contents--Cummings says it contained instructions on further information-gathering--a warrantless wiretap was placed on Truong's telephone. And that, in turn, led to Humphrey.

"Really, during the whole Vietnam period I assumed I was watched," says Truong. "I didn't question Krall's motives until very much later when she was pretty nervous about what she did.

"This is just like a rerun of the war in miniature," he adds. "The CIA, which was running her, had an agent that was, in every sense of the word, greedy for money and the agent basically destroyed herself in the process. In Vietnam, you know, family and some type of moral values are very important. I think she lost pretty much all that."

The Justice Department said Krall, the wife of an American naval officer, who would not agree to testify at the trial until questions of her espionage pay were resolved, has left the Washington area and was unavailable for comment.

"This case is a funny case because you can write it from one of two angles and you could be very convinced both ways," says former prosecutor Cummings, now in private practice in Alexandria. " Truong's story is not that unpersuasive. This was not a guy who was out, I'm convinced, to do in the government of the United States. And we never contended that he was. He was not a guy who was hoping to get the atom bomb to Hanoi so they could kill us all.

"On the other hand, he clearly knew that what he was taking was clandestine and secretive."

While Humphrey has worked since the trial running a summer camp and recreation programs for Calvary Baptist Church, 755 8th St. NW, Truong has traveled the country giving speeches and raising money to pay his legal costs.

He is soft-spoken and articulate, with apparently strong convictions about the future of Vietnam and his own prospects after prison.

"I think at this point, if given the choice, I probably kind of would like to leave the United States ," he says. "I'm not going to contribute anything further over here in terms of helping the normalization take place because that's way down in the priorities. It's time to let the dust settle where it has to settle."

He has considered resettling in France, and wants to visit his parents in Vietnam. His father, once an affluent Saigon lawyer, spent five years at hard labor after his unsuccessful bid to replace then-President Thieu in 1967. He is now living in retirement in Ho Chi Minh City, partially crippled, Truong says, as a result of his treatment in prison.

"From one generation to the other in my family everyone has been in jail for political reasons," he says.

When he last talked to Humphrey, at the trial, Truong says he told him: "I just hope he--he has four half-Vietnamese children, you know--I told him I hoped that he and his wife would be able to raise their children so they would not be against what has happened, or against Vietnam . . . To raise kids like that is not easy because they have seen the war . . . I was more or less telling him that he should not, if he could avoid it, raise kids that would continue to fight that same old war forever."

For his part, Humphrey spent the final days of freedom preparing his household "as if I were terminally ill," he says, giving some driving lessons to the family, filing his income tax, tutoring the children in English. Under federal prison guidelines both he and Truong will have to serve at least five years in order to be eligible for parole.

Eight days ago the Humphreys were driving home from church and crossed the 14th Street bridge as a wing of the crashed Air Florida jet was being raised from the Potomac River. "Daddy," Humphrey says his 12-year-old daughter cried, "What if you'd been on that airplane. Then you'd never be coming back to us."