
The story is very interesting, unusual and comic. At first it seems to the audience that it's just a show and nothing more but if you imagine Russia of those years you can easily realize that the story could really happen:

Some friends meet at a Moscow's banya to celebrate New Year's Eve (we call it Novy God). All of them get very drunk, and two of them, including the main character, Zhenya pass out. The others forget which of their unconscious friends was meant to be catching a plane to Leningrad (now St. Petersburg), and so Zhenya is put on the plane by accident instead of his friend. He wakes up at Leningrad airport, believing he is still in Moscow. He gets into a taxi and gives the driver his address. It turns out that there exists a street in Leningrad with the same name, and a building which looks exactly like Zhenya's. The key fits in the door of the apartment, and even the layout of the apartment is the same. Zhenya is too drunk to notice any difference, and goes to sleep. Later, Nadya, who lives in the apartment, comes home and finds a man she's never met before asleep in her bed. To make things worse, Nadya's fiance Ippolit, arrives before Nadya can convince Zhenya to wake up and leave. Zhenya desperately tries to get back to Moscow in time to spend New Year's Eve with his fiance Galya, and Nadya wants to get him out as fast as possible, but unfortunately there are no flights to Moscow for some time. The plot starts out as a comedy, but becomes more dramatic as it evolves around the relationship of Nadya and Zhenya, along with their relationships with their fiancees. Initially Nadya and Zhenya dislike each other intensely, but they eventually fall in love during the course of the movie. An important note about the coincidence with the addresses: many street names are/were common to Soviet and now Russian cities (for example, Sotsialisticheskaya, Lenin Street, etc.). Many houses look identical, and even apartments look very much the same from the inside. Thus, e.g. nobody has to ask for directions to the toilet, because the toilet is always next to the kitchen, and knives are always in the same drawer in the same cupboard that was built in all apartments of a certain type. The matching key is probably a joke by the film makers, though many Soviet locks look very much the same. The epoch distinguished by the shortage of goods, that's why you could always see the same glasses, the same furniture in every flat.
It's difficult to believe but you don't get tired of seeing this film again and again. I can't imagine the typical day of December 31st without it. There are films that are eternal. «Irony of Fate» is one of them.
(wikipedia.com, lenta.ru)