ON FEB. 11, 1986, at precisely 10:58 a.m., Natan Sharansky, wearing a fur hat, a black overcoat and baggy pants, crossed the Glienicker Bridge from East Germany to West Berlin and freedom.

Half an hour later, another man and his wife strode over the bridge in the opposite direction. With his mustache and fur-lined coat, Karl F. Koecher looked like nothing so much as a fox. His wife, Hana, wore a mink coat and high white mink hat. Blond and sexy, with incredibly large blue eyes, she looked like a movie star.

Press accounts that day described Koecher as a member of the Czechoslovak Intelligence Service who had worked for the CIA from 1973 to 1977. Because of the shroud of secrecy surrounding his case, very little was known about him beyond that.

The CIA had wanted it that way. After all, Koecher was the only spy known to have penetrated the CIA on behalf of the Soviet KGB. He had continued as a CIA contract employee until the FBI arrested him and his wife in 1984. In intelligence circles, Koecher is known as one of the most important spies in U.S. history.

What follows is Koecher's story, told in detail for the first time. Together with his wife, Koecher or- chestrated a phony defection from Czechoslovakia in 1965 and developed a "legend" or cover story to conceal his spying activities in New York and Washington. He obtained a job as a CIA translator and analyst in 1973, managed to meet his Czech and KGB handlers at safe houses in Austria and Zurich and continued to pass classified documents to the Czechs and KGB as a CIA contract employee until the FBI caught up with him after 20 years of spying.

Koecher's case is thus a classic spy success story, one that earned him multiple decorations from the Czechoslovak Intelligence Service and the KGB. He was the prototypical "illegal," an agent of a hostile intelligence service who infiltrates another country's intelligence service without ever appearing to have any ties with his original employer.

"If you look at this operation, you'd be hard-pressed to find anything as professionally done," according to James E. Nolan Jr., who was deputy assistant FBI director for operations when the FBI first got onto Koecher.

The Soviets showed their appreciation by trading Koecher for one of the most prominent Soviet dissidents of all time. But there was more to the Koecher case than that:

Koecher gave his Czech handlers and the KGB details of dozens of "top secret" CIA operations targeted at the Soviets and U.S. allies alike. He supplied them with classified CIA documents, lists and photographs of CIA employees in the United States and overseas and names of U.S. government officials who might be blackmailed into cooperating with the Soviets.

One of the CIA documents Koecher gave up compromised Aleksandr D. Ogorodnik, a Soviet diplomat in Moscow who was a key CIA asset. Upon being confronted by the KGB in 1977, Ogorodnik committed suicide.

The Koechers led still another secret life -- that of swingers who attended spouse-swapping parties, orgies and free-for-alls at sex clubs like Plato's Retreat in New York. Many of the parties were attended by other CIA employees; Koecher passed their names along to the KGB for possible recruitment.

In a bizarre twist, New York FBI agents, together with a CIA official, eventually obtained Koecher's confession using illegal methods: making promises that they never intended to keep and threatening the personal safety of Koecher and his family. Secretly, Justice Department officials reprimanded the FBI for disregarding all guidelines governing confessions, and the FBI's Office of Professional Responsibility censured the agents.

I was curious to meet this superspy, so just over a year after the historic trade on the Glienicker Bridge, I called a relative of Koecher's in Czechoslovakia. Two days later, Koecher called me back. During a 10-minute conversation, he agreed to be interviewed, but only in Prague. Because of the way he had been treated, he feared being kidnaped if he entered the West again.

The Koechers met my wife and me on April 29, 1987, at the Prague airport, a modest spot in the rolling green hills of Czechoslovakia. He fussed with our bags to make sure they fit in the trunk of his gray 1986 Volvo.

There was nothing about the Koechers that would lead one to believe they were spies, at least not at first. He was dressed in Brooks Brothers fashion: gray tweed suit, blue shirt, red tie. Hana wore a tawny leather jacket, a gray tweed skirt and an expensive white blouse with lace trim.

Fifty-three years old, Karl was slight of build but muscular, with greying light-brown hair cut short and pale brown eyes. His head was irregularly shaped, as if his ears were pushed into it. His long nose curled down over his mustache. When he smiled, the tops of his cheeks crinkled.

Hana, 43, wore a lot of jewelry -- a heavy gold chain, a wide bangle and a large diamond wedding ring. Her hair was streaked blond. Her hands were somewhat pudgy. Her lips were full and sensual.

Making no secret of the fact that he was a spy, Koecher told me that no Soviet spy had ever agreed to be interviewed by the western press before, and he explained why he had agreed to this first:

"Even if it comes out principally hostile, I can only gain by talking to you," he said. "It will certainly be somewhat less hostile simply because it was not really as described in the publicity. A serious writer for the sake of his own prestige would try to do an honest job."

He indicated he still works for the Czechoslovak Intelligence Service, where he is targeted at the United States. His share of our meals -- we took turns paying -- went on his expense account. At one point, when three men seemed to be listening to our conversation, he suggested we move. I asked him who they were.

"They're not ours," he said quite seriously. "They must be yours."

Over the next five days, we toured Prague and its restaurants, ate wild pheasant and deer sausage, visited the Old Jewish Cemetery, saw the concert hall where Mozart's "Don Giovanni" was first performed and viewed Czech art at the National Gallery. One day we drank slivovitz, a strong plum brandy, in the Koechers' new home, walked in the woods and sampled Hana's cooking.

All the while, Koecher and I taped and photographed each other while we probed each other for hidden agendas. I learned how Koecher had checked me out before I arrived and how at one point during our visit he was convinced my wife was an undercover FBI agent. He discussed their swinging activities and all the changes in their lives now that they are in Czechoslovakia, barred from ever again entering the United States.

Koecher spoke, too, of his Jewish heritage, how his mother had hidden her Jewish ancestry to escape persecution during the Nazi occupation of Czechoslovakia and how his grandparents were killed in a Nazi concentration camp. I even met Koecher's 88-year-old mother, who had lived for 20 years in the belief that her only child had genuinely defected.

Most of all, Koecher talked about what he referred to as his "career" as a spy, why he chose it and likes it and his experience in the United States as an "illegal."

In early 1982, the counterintelligence squad that handles Czech matters in the FBI's Washington field office got wind of Karl Koecher's activities on behalf of the Czechoslovak Intelligence Service and the KGB, which generally operate together. Conducting routine surveillance of Czech intelligence officers, the FBI discovered that they were having "brush contacts" or "brief encounters" with a man of slight build who had a graying mustache, an angular chin and close-cropped brown hair.

Tailing him, the FBI determined that the man who was passing documents to the Czech agents in these surreptitious meetings was a Czech who had become a naturalized U.S. citizen in 1971. Delving into his background, the agents discovered that Koecher was a CIA contract employee. His wife, Hana, had emigrated to the U.S. with him. It was not long before the FBI found that the attractive, blue-eyed blond was part of the spy team, filling "dead drops" for her husband and making brush contacts with Czech agents to pass information or documents. Moreover, the FBI was astonished to find that Koecher in the early 1970s had almost become a double agent for the FBI.

There was a lot about Koecher that didn't make sense. Presumably, a spy whose mission in life is to remain undetected would lead the most prosaic, unblemished existence imaginable for fear of attracting attention. Not so Karl Koecher. To his friends, employers and the philosophy students he taught when he was an assistant professor at two New York colleges, he presented himself as a virulent anti-communist. So seething was his apparent hatred for the rulers of his homeland that he tried to blackball tennis star Ivan Lendl from buying an apartment in his East Side building in New York -- simply because Lendl came from Czechoslovakia.

To some of his friends, Koecher seemed to be Jewish, but others thought he was an anti-Semite. On job applications he claimed to be Roman Catholic.

Perhaps the most unusual behavior for a mole wanting to remain under deep cover was Koecher's penchant for swinging. At least once or twice a week, Koecher and Hana had one or two couples over for dinner, or went to their homes, and swapped spouses for sex. While they had a wide circle of friends for the purpose, the Koechers also participated on a regular basis in larger sex parties and orgies in New York and Washington, which Koecher told me was the "sex capital of the world." They frequented a nudist colony in New Jersey, along with Plato's Retreat and the Hellfire, two sex emporiums in New York that were open to anyone who could pay the price of admission.

In retrospect, Koecher's exotic lifestyle and peccadillos, his outbursts and seeming instability, were not outlandish but rather brilliant. For who would suspect that a master spy would be so peculiar and perplexing? And how does one extract truth from one who appeared to be an enigma even to himself? Sometimes it seemed Koecher must be schizophrenic, for how else could he so convincingly pass himself off as someone he was not?

Much about Koecher will always remain hidden, wrapped in layers of deceit that even he could not unfold if he chose to. While he admitted in 1984 to doing incalculable damage to the CIA and its assets and later pleaded guilty to espionage as part of the exchange agreement, the FBI is convinced that the full story will never be told. But from secret FBI interviews and electronic intercepts, interviews with the Koechers' friends and former employers, copies of the couple's diaries, letters and job applications and interviews in Prague with the spy couple themselves, it is possible to gain some idea of who Koecher is and how he managed to fool the CIA.

In developing a "legend" to conceal their activities, "illegals" typically adhere to most of the facts from their past so they won't become confused, omitting only those details that would point to their true allegiance. So it was with Koecher. Undoubtedly, the resume that he submitted to the CIA in 1972 was true as far as it went -- it simply failed to state that he joined the Communist Party in 1960 and that, in 1962, the Czechoslovak Intelligence Service began training him to get a job with the CIA.

After his three years of training, the Koechers defected to Austria while visiting a relative. They entered the United States as immigrants on Dec. 4, 1965. To build his resume, Koecher studied at Indiana University's graduate school. He then moved to New York, where he transferred to Columbia University. His professors included Zbigniew Brzezinski, who later became President Carter's national security adviser.

By 1972, Koecher's "legend" was established. He was ready to carry out the assignment he had received 10 years earlier. Now an American citizen, he was an established member of the U.S. academic community with a special interest in Soviet affairs. He could list impressive references from well-respected scholars in the field. And anyone who checked with his employers or friends would find he was violently anti-communist.

Koecher passed the FBI's background investigation and, on Oct. 30, 1972, took a CIA lie-detector test. Asked during the test if he had ever been a member of the Communist Party, he said he applied in 1959 to get ahead in his career in Czechoslovakia but had been turned down. Koecher passed the test. Later, during the FBI's investigation, polygraph experts noticed that the CIA had misread the test results.

Koecher became a CIA translator with "top secret" security clearance on Feb. 5, 1973. He was assigned to AE/SCREEN, a cryptonym for a translation and analysis unit in the CIA's Soviet section. From a rented building in Rosslyn, just across Key Bridge from Washington, he translated reports or tapes dictated in Russian or Czech by CIA informants. He also translated and analyzed intercepted conversations. Because of his knowledge of science and engineering terms, he was given some of the most sensitive material the CIA had.

One of Koecher's CIA assignments was to translate reports from Aleksandr Ogorodnik, the key CIA asset in Moscow mentioned above, and to write an evaluation of him. In 1974, while serving at the Soviet embassy in Bogota, Colombia, Ogorodnik had made contact with the CIA. Over a period of 20 months, after he was transferred to the Global Affairs Department of the Soviet Ministry of Foreign Affairs in Moscow, he provided the CIA with microfilm of hundreds of classified Soviet documents, including reports from Soviet ambassadors.

The information was so valuable that it was circulated, in summary form, to the White House, National Security Council and State Department.

The KGB caught Ogorodnik photographing documents in 1977 and arrested him. As recounted by John Barron in "KGB Today," Ogorodnik confessed immediately, saying he was prepared to pay the price for acting on his political beliefs. He then asked for a fountain pen from his apartment so he could write a confession.

The KGB delivered the pen, which contained poison carefully concealed there by the CIA. He opened it, swallowed a pill and died within 10 seconds. Ronald Kessler, a former Washington Post reporter, is the author of the forthcoming "Spy Vs. Spy: Tracking Soviet Spies in America," from which this article is adapted.