Almost exactly a year ago a Vietnamese woman code-named "Keyseat" by her superiors at the FBI and CIA arrived in Paris carrying a flight bag filled with at least 49 classified U.S. State Department documents.

Two days later she turned the documents over to representatives of the Hanoi government, who thought she was working as a spy for them.

The Vietnamese in Paris were interested in the documents because they were just about to start talks with the Carter administration about U.S.-Vietnamese were cooperating in the identification of remains of Americans missing in action, and they expected the United States to support their admission to the United Nations. The mood was hopeful.

But throughout April, May and June of 1977, as the diplomatic negotiations in Paris held the attention of the press, another quite secret drama involving the Americans and the Vietnamese was being played out on the streets of Washington.

Furtive meetings in a Dupont Circle cafeteria, a suburban shopping mall and the home of a well-known Vietnamese expatriate in Washington soon would come to the attention of President Carter and the highest officials of his administration.

Because of the way they chose to investigate what they believed to be a serious case of espionage, important constitutional questions have been raised, Attorney General Griffin Bell went to court to defend his actions and considerable attention has been focused on a trial, which is scheduled to begin May 1 in federal court in Alexandria. It will be one of the very few espionage cases to be tried in recent years and the only one to grow out of this nation's long involvement with Vietnam.

The attorney general first learned of the case the day after the opening round of talks in Paris was over, when he received urgent phone calls from Deputy Secretary of State Warren Christopher and FBI Director Clarence Kelley.

They arranged a meeting for the next day, where Bell was told of an FBI operation called "Magic Dragon," a counterintelligence investigation of Vietnamese antiwar activist David Troung whose father ran as a peace candidate against Vietnamese President Thieu in 1967 and subsequently was jailed. Truong was known among antiwar legislators on Capitol Hill as a respected source of information about what was "really" going on in Vietnam during the war.

"Magic Dragon," Bell was told, had uncovered "one of the worst leaks in State Department history."

The attorney general said in court recently that he was asked during that April meeting to help find out just who was taking the documents, and exactly where they were coming from. He said he was told they might endanger the lives of foreign embassy personnel in Hanoi who had supplied the United States with information.

Bell said he was now shown the documents during the meeting, and according to an FBI affidavit it was not known at the time how many were classified - how secret they were or were not - because whoever had taken them removed such information.

But Bell said recently he was impressed with the gravity of the situation as Christopher and the FBI presented it. "I thought in my own limited way," he said, "that something had been done wrong to our country."

He was asked, he said, to "prevent and detect" the leaks. And one way to do it, he was told, was to authorize a warrantless wiretap on Truong's telephone.

It was Friday, May 6, and the attorney general was getting ready for a leisurely weekend in Louisville for the Kentucky Derby. He had been for some time, in the middle of several complicated problems that would be complicated even more by such a case.

"When I became attorney general, he said in court, "I didn't know the attorney general engaged in counterintelligence activities."

Since taking in office, however, he discovered he was not only involved, but often on the defensive caught up in the conflict between the duty to protect the nation from espionage and the people's Fourth Amendment right to privacy.

The debate over a president's "inherent power" to authorize searches and wiretaps without a court order, raised during Watergate, still raged in the headlines.

Even as the Carter administration was unveiling legislation to require secret warrants in such cases a retired FBI agent in New York was indicted for illegal wiretaps and burglaries conducted in 1972. (The investigation of that case would eventually lead to rifts in the Justice Department and the indictmen of former acting FBI director L. Patrick Gray III.

Bell decided to go to Louisville for the weekend. But on the day he came back to Washington he used the "inheren tpewer "delegated to him by the president to authorize a tap on Truong's telephone.

Bell also decided to pay a personal visit to the president to discuss the case. "Keyseat" would be coming back soon with a letter to Truong from a Vietnamese official in Paris, and Bell got the president to authorize its opening.

"I applaud your business plans," it said, according to an FBI translation, but it warned that "very often there are efforts to swindle and deceive one another."

According to court records the wiretap on Truong's phone went into operation in May 11 and ran continuously until after he was arrested. In all, more than 550 conversations were recorded, dealing with everything from "Star Wars" to the subtleties of Vietnamese cooking.

But within two days after the tape started rolling - on May 13 at 6:33 p.m. - the FBI had, essentially, what it wanted: Truong was overheard asking someone named Ron to stop by his apartment, and when a pale, baldish man arrived there a little after 10 that night the FBI was watching.

When the man came out of the gray building on F Street about an hour later, an FBI agent followed him. He walked through the wide, dark streets near George Washington University and past the World Bank to the United States Information Agency building a few blocks away.

The agent watched as the elevator indicator in the USIA lobby counted its way up to the seventh floor. The agent then checked the sign-in register where he found the name: "R. Humphrey."

Soon, Bell was asked for another authorization - to place, without a court warrant, a listening device in Truoun's apartment, in case there were secrets being discussed there. After carefully going over the methods that would be used to plant it, Bell approved the microphone.

After May 27, every sound made in Truong's apartment was overheard by the FBI.

Finally the president authorized televsion surveillance in the USIA office thought to be the source of the leaks. From June 15 until end of the summer every move made there was observed, according to court testimony, by two video lenses peering down from the ceiling.

The object of their attention was Ronald Louis Humphrey - later to be indicated as Truong's codefendant. But at the time, according to his lawyers, he was mostly preoccupied with efforts to get his Vietnamese commonlaw wife and stepchildren out of Vietnam, where they had been trapped since the fall of Saigon.

Humphrey secured their release just a month after the FBI told the attorney general that federal agents had found the man who could be taking the documents - classified cables, many of which Humphrey had access to as a night "wtch officer" in the USIA communications room.

After the FBI began to concentrate its investigation on Humphrey, the question became, Bell said, how to stop the leaks.

According to FBI agents there are a number of different ways to handle such a case.

An alleged spy might have been put in a position where access to classified material was limited.

Or a suspected spy might be "doubled" - persuaded to work for United States counterintelligence (Humphrey's lawyer says that Humphrey did cooperate in the investigation of an alleged Russian agent late in 1976.)

These or similar measures, if they had been possible, would have gone relatively unnoticed. Another way to proceed, however, was criminal prosecution, and during the summer of 1977, though there were many obstacles and objections, that is the course Bell decided to take.

The Supreme Court has never ruled on the use of warrantless electronic surveillance in a foreign intelligence investigation, and the District of Columbia's U.S. Circuit Court of Appeals has questioned its legality.

Federal courts in Virginia, however, generally have expressed fewer reservations on such issues.

On June 22 "Keyseat" arranged to meet Truong at the Landmark Shopping Center in Alexandria, according to her testimony in court. She said she didn't want to go too far from her home in Springfield - where she was making preparations for a move to London - and in any case she was afraid to drive around Washington at night.

Truong borrowed a car to get there, and as three FBI agents took pictures from a distance he gave "Keyseat" a grocery bag full of papers, the court was told.

Inside the bag, according to court records, were 33 more State Department documents, 28 of them clearly labeled confidential, and one Defense Intelligence Agency paper marked "secret," according to government inventories.

It is some of these documents that the government used as a basis for the present indictment of Truong and Humphrey. The reason, according to prosecutors, was to make sure the case would be tried in Virginia.

Regardless of where it came to court, however, Bell knew that he could not make an epsionage case against Truong or Humprehy without revealing the contents of some or all of the documents involved.

Because of her intimate involvement with the investigation, he also felt he would have to have the testmony of "Keyseat."

Her civilian name is Dung Krall. Her father is a former Vietcong ambassador to the Soviet Union, but for more than a decade she has been married to an American who recently indentified himself as a naval intelligence officer.

Over the summer, as Bell sought to get the release of the documents and the court testimony of "Keyseat," one prosecutor has said he met with "tremendous opposition" from within the State Department and the National Security Council. The CIA, meanwhile, was not about to give up its agent.

Early one August afternoon, Bell drove out out to Langley to talk with CIA Director Stansfield Turner about "Keyseat." Soon afterwaed her FBI and CIA case officers flew to London to talk to her about testifying.

She saud she would think about it.

Meanwhile, however, Vietnamese-American relatives had begun to worsen.

Thought the United States agreed to back Vietnam's admission as a full member of the United Nations after the negotiating sessions in May and June, and there was talk of exchanging diplomatic missions, the question of money Hanoi said it was promised in 1973 to "heal the wounds of war" remained unresolved. The Vietnahese insisted on it, and the United States refused to budget.

At about the same time as the December talks in Paris, the State Department gave the "necessary approvals" for prosecution of Truong and Humprey, according to an affidavit signed by Bell.

During the last week of January 1978, after the government agreed to pay $11,800 for her relocation and protection, in addition to her monthly fee of $1,200, Keyseat agrees to testify.

Jan. 31, Truong and Humprey were arrested and charged with crimes of espionage that could put them in jail for three lifetimes.

The indictment named Vietnamese embassy officials in Paris, and Dinh BaThi, the Vietnamese ambassador to the United Nations, as unindicted co-conspirators. Within days Thi was ordered out of the country - a move that had never before been taken against a U.N. ambassador.

To further complicate matters at the State Department, nearly 20 foreign nations had to be told that the same documents Truong and Humprey are accused of stealing - which allegedly compromise foreign embassy personnel in Hanoi - may be made public during the trial, and in most cases were knowingly handed over to the Vietnamese by "Keyseat."

The day Truong was arrested, FI agents fanned out across the country to interrogate people who had been overheard on his telephone, were his close friends, or who might have information pertinent to the case.

At the Justice Department there is a story about Bell's mood on the day of the arrests.

People who were at a staff breakfast that morning now say they cannot remember exactly what was said. "These things into the breakfast in a great humor. "Today, boys," he is supposed to have told them, "we're going to catch us a real spy."