The Long Farewell (1971)

A single woman has put all her efforts into raising her only son, Sasha. When Sasha grew up to become a teenager, he got tired of the nagging of her mother. One summer, he goes to visit his father in Novosibirsk, on the other side of the country. When he returns, his mother notices that Sasha had changed. She secretly reads a letter that Sasha received from his father and she finds out that Sasha doesn't want to live with her any longer. She cannot understand, she cannot accept. When Sasha realizes that she is suffering because of his wish to leave, he decides to stay.