The Elder Son (1975)

Original title: Старший сын

Two students are late for the last commuter train and have to spend a night in a strange town. They rack their brains about where to find shelter and quite by chance they overhear a talk between father and son. The son is an adolescent who is head over heels in love with a girl several years his senior. He is living through the first drama of his life though his love is just puppy love. Then, being resourceful fellows, they think of a plan to pass one of them for the elder son of this family. The reason they give for his unexpected arrival is that he is a child of the father's long-forsaken love. They presume this cock-and-bull story will come off, and right they are!