Boris, Sonia and their son Den live in a modest "Khruschev-era" apartment in Moscow. The guy is turning eighteen soon, but he won't pull himself together - he's strumming the guitar for days on end and composing songs. Sonia is teaching Russian language and literature at school, running a theatrical study group, and moreover moonlighting as a tutor. Whereas Gleb Dorokhov's family lives in Viyazov. Gleb is married to Sonia's sister Raisa. Our story begins when Marina arrives in Moscow, and Den in Viyazov. There are a lot of reasons for that. Marina should enter university and study. And it's high time Den pulled himself together - Sonya hopes that Gleb will keep him busy. Everything would be fine, but it seems that there is a mystery that has been connecting two families for years. And the reason which urged parents to exchange the children, are not those that seem to be.