It is based on a true story. On December 26th 1980 a KGB agent was found dead on his birthday. He was last seen alive on a subway station which was in the opposite direction than his home from the center. The KGB suspected that maybe he was killed by foreign agents, after previously that year another agent disappeared on the same subway station. They launched an investigation, which reached shocking conclusions. The names and some of the facts were changed in the film due to censorship and for cinematic reasons, but the core facts and story is true. The ending was quite different, though. In reality the culprits were tried and sentenced to death. The Minister of Internal Affairs was fired and eventually committed suicide. By the way, after about 15 years, it was discovered that the other missing agent actually defected to the USA with his family, and had no connection to this case, except for his disappearing on the same station.