The key subplot is the drab uniformity of Brezhnev-era public architecture. This setting is explained in a humorous animated prologue, in which architects are overruled by politicians and red tape (director and animator - Vitaliy Peskov). As a result, the identical, functional but unimaginative multistory apartment buildings found their way into every city, town, and suburb across the former Soviet Union. These buildings are completely uniform in every detail including the door key of each apartment.
Following their annual tradition, a group of friends meet at a banya (a traditional public "sauna" bath) in Moscow to celebrate New Year's Eve. The friends all get very drunk toasting the upcoming marriage of the central male character, Zhenya Lukashin (Andrei Myagkov) to Galya (Olga Naumenko). After the bath, one of the friends, Pavlik (Aleksandr Shirvindt), has to catch a plane to Leningrad. Zhenya, on the other hand, is supposed to go home to celebrate New Year's Eve with his fiancée. Both Zhenya and Pavlik pass out. The remaining friends cannot remember which person from their group is supposed to catch the plane -thus they mistakenly send Zhenya on the plane instead of Pavlik.
Zhenya spends the entire flight sleeping on the shoulder of his annoyed seatmate, played by the director himself (Ryazanov) in a brief comedic cameo appearance. The seatmate helps Zhenya get off the plane in Leningrad. Zhenya wakes up in the Leningrad airport, believing he is still in Moscow. He stumbles into a taxi and, still quite drunk, gives the driver his address. It turns out that in Leningrad there is a street with the same name (3rd Builders' street), with a building at his address which looks exactly like Zhenya's. The key fits in the door of the apartment with the same number (as alluded to in the introductory narration, "...building standard apartments with standard locks"). Inside, even the furniture is nearly identical to that of Zhenya's apartment. Zhenya is too drunk to notice any minor differences, and goes to sleep.
Later, the real tenant, Nadya Shevelyova (Barbara Brylska), arrives home to find a strange man sleeping in her bed. To make matters worse, Nadya’s fiancé, Ippolit (Yuri Yakovlev), arrives before Nadya can convince Zhenya to get up and leave. Ippolit becomes furious, refuses to believe Zhenya and Nadya's explanations, and storms out. Zhenya leaves to get back to Moscow but circumstances make him return repeatedly. Nadya wants to get rid of him as soon as possible, but there are no flights to Moscow until the next morning. Additionally, Zhenya tries repeatedly to call Moscow and explain to Galya what has happened. Eventually, he does contact her, but she is furious and hangs up on his call. Ippolit also calls Nadya's apartment and hears Zhenya answer. Although Zhenya is trying to be available to receive potential calls from Galya, Ippolit also refuses to accept the truth of the situation. It seems more and more clear that Zhenya and Nadya are the only two people who understand the night's circumstances.
Thus, Zhenya and Nadya are compelled to spend New Year's Eve together. At first, they continue to treat each other with animosity, but gradually their behaviour softens and the two fall in love. In the morning, they feel that everything that has happened to them was a delusion, and they make the difficult decision to part. With a heavy heart, Zhenya returns to Moscow. Meanwhile, Nadya reconsiders everything and, deciding that she might have let her chance at happiness slip away, takes a plane to Moscow to find Zhenya. She has no difficulty finding him as their addresses are the same.
