In the year since a Soviet fighter downed a Korean passenger plane over the Sea of Japan, the Soviet version of the event has remained unchanged.

The Soviets have offered no apology and no admission of a mistake. Instead, they have taken every opportunity to blame the incident on an alleged American spy mission -- a theme hammered home again in recent articles apparently timed to counteract the marking of the Sept. 1 anniversary in the western press.

The decision to shoot down the plane was a vivid illustration of this country's defensiveness about its borders. The handling of its aftermath revealed another kind of defensiveness, as the government media tried to deflect the international outcry over Soviet actions.

"Who is guilty of destroying the airplane?" asked a headline in the Aug. 18 edition of Pravda, the Communist Party daily. The answer, according to Pravda, is Washington -- the same answer given by Marshal Nikolai Ogarkov at an unusual news conference about Korean Air Lines Flight 007 last September.

Recent Soviet articles about the downing of the jumbo jet have relied heavily on western accounts questioning the Reagan administration's version of the event, including a recent West German documentary, articles in The Nation and in the British publication Defense Attache, in Japanese and Italian newspapers.

Over the weekend, Radio Moscow reported reaction in Brazil to a story in an Italian newspaper quoting a former U.S. diplomat as claiming the United States blew up the jet with a remote-controlled bomb to conceal an abortive spying mission. The one-time diplomat, however, assailed the report, saying that while he believed there was an explosion aboard the aircraft, he had never indicated sabotage as its cause.

Not only was Flight 007 on a spy mission, but the incident, "invented and planned in Washington," was then used "to incite anti-Soviet hysteria and to increase the arms race," Pravda quoted other papers as saying.

An article in Wednesday's Literaturnaya Gazeta, on a page devoted to Flight 007, touched on the same themes and concluded: "Why now should one come back to the events of last fall and their consequences? In November, a president and vice president of the country will be elected . . . Ronald Reagan and his Californian team are doing everything in order to hang on in Washington and they will not shun any means. . . . That's why for the coming weeks and months one should watch with particular caution for any hostile statements or 'jokes' coming out of Washington and for any provocative tricks that could be plotted there."

This pattern of commentary continued tonight, when the official news agency Tass carried a report on an open letter to President Reagan from a group of Japanese claiming the United States was at fault in the downing of the airliner.

Many Soviet citizens appear to accept their government's version at face value, although enough of them have questioned westerners about what "really" happened to Flight 007 and its 269 passengers and crew to suggest considerable doubts. As usual, Soviet citizens have ready access to the official version of the event, but have difficulty learning about others.

While the official version has admitted to no second thoughts about the downing of the Korean plane, it appears that the episode did have repercussions within the Soviet Union's ruling circles.

The Washington Post reported last October that several senior officers of the Soviet Far East military command were removed from their jobs last fall for failing to locate the plane during its flight over the Kamchatka Peninsula.

At the time, then-Soviet president Yuri Andropov's invisibility at the height of the crisis raised questions about his ability to handle and control Soviet reaction. In hindsight, his illness -- then unknown -- may offer at least a partial explanation for his initial silence, although there is still speculation that the nation's political leadership tried to distance itself from what was perceived as a problem created by the military.

A recent article in the London Observer suggested that the Soviet leadership has attempted to address concerns about crisis management raised by the incident.

In the aftermath of the downing of Flight 007, "Andropov is believed to have set up, and his successor Konstantin Chernenko to have retained, a routine by which at least one Politburo member must be at the end of a telephone line to take on-the-spot decisions when the top leadership is momentarily out of reach," the Observer concluded.

However, this appeared primarily to be surmise. Details of the workings of the top level of the Soviet government are almost never revealed to foreigners.

The downing of Flight 007 marked a sharp decline in U.S.-Soviet relations, although it was certainly not the only cause, diplomats here say. It since has been overtaken by other events, including the deployment of NATO missiles in Western Europe and the breakdown in arms control talks, but the downing came at a key time: bilateral talks were underway on low-level issues, Soviet Foreign Minister Andrei Gromyko and Secretary of State George P. Shultz were scheduled to meet at the United Nations, and arms control delegations were heading for Geneva for new rounds of talks.

"The great fear," said the London Times last Sept. 2, "must be that yesterday's incident will put back the clock."

It was not long before that fear was realized. The negotiations on a cultural agreement and consulate exchanges were put on hold, the meeting between Shultz and Gromyko was canceled when Gromyko's plane was not given landing rights and on Sept. 28, Andropov in a major pronouncement delivered a scathing attack on U.S. foreign policy. He parenthetically blamed Washington for the incident, which he called a "a sophisticated provocation masterminded by U.S. special services."

"With the benefit of a year's hindsight, the shock has worn off on people, although it lingers as confirmation that these are people who play hardball," a U.S. diplomat said here recently. "It set back things that were on track in U.S.-Soviet relations, but that was more of the moment. Now we are trying to find areas to improve [the relationship]."