Love can not be divided into 2

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. Not immediately, but gradually, the viewer begins to realize that Sonia and Gleb are real parents of Marina and Den. But how could it turn out that for as long as eighteen years Sonia was separated from her daughter and Gleb from his son? It is to be found out only in the very finale.