Parents of thirteen years old Lena just got divorced. Lena wants to prove that they live nicely even with her mother alone and that she is independent. The girl goes to work at post office utilizing there her resourcefulness and charm. At work everyone talks about a serial killer who keeps the entire region in terror. One night a stranger defends Lena from some hooligans and as Lena thinks, he continues assisting her incognito. The girl is ready to believe and convince others that her protector is that horrible maniac. At first, these relationships with her odd admirer seem as a gamble for Lena, but then she gets convinced that this game became deadly dangerous...

The film is a reflection on the responsibility of man to other people, native nature and the world around us, the problem of preserving Lake Baikal, on the banks of which a large construction project has unfolded. The first half of the 60s of the XX century, the story of the love of young Lena Barmina to an adult respectable person, bearing on his shoulders a heavy burden of responsibility for the activities of a huge enterprise. Construction engineer Chernykh and Lena, the daughter of a scientist, are doing everything to save the lake from harmful and unjustified plums. A difficult relationship develops between them and soon Lena realizes that she is in love. But after learning that Chernykh is married, the girl leaves...