This film is almost unbearably sad and eventually tragic. It is about a young Russian war widow who, on a crowded train in the mid-to-late 1940's, meets a man. The man seems to be a young Red Army officer, but in fact is a thief dressed in a way likely to aid his movement about Russia in Stalin's day (when people could not even board a long distance train without frequently showing an internal passport --which system carried on until the 1980's and seems totalitarian; but then look at today and at the criminality which has flourished now the reins have been loosened).
I do not want to spoil anyone's enjoyment, but I think that in this case it is OK to give a synopsis of the plot.
In a very sad moment, the young son of the widow sees his real father (obviously lost in the Great Patriotic War of 1941-45) as a shadowy and longed-for figure in the darkness around the moving train.
The young woman and officer (now living as husband and wife, in effect) and young boy start living en famille. She discovers he is a thief and is horrified but loves him. The boy idolizes him and, although the thief is hard on him at times, also helps him to grow up and protects him. The thief (now without Army uniform after a near-arrest) tells the boy that he, the thief, is actually working on a secret project for Stalin.
Living in a Soviet-bourgeois area (either Sochi or Yalta) with a beach and a warm climate, eventually pigeons come home to roost: the thief is arrested. The mother of the boy dies, tragically, while the boy is placed in a fairly harsh looking institution for orphans and foundlings etc.
As a teenager, the boy has hidden a handgun. One day he sees the thief, now released, drinking with a group of drunks and a Sovyetskaya devka ("floozy")! As the thief gets onto a moving goods train, the teenage boy shoots him and sees the thief fall.
The boy goes on to become a commander of field rank in the war in the Caucasus, as the Soviet "empire" falls apart. An old man, drunk, falls at his feet, dying. It is the thief, who was not killed at all, all those years before. The commander, shocked to the core, gives up his most of his command coach on the train to women with little children. As the train pulls out, he sees rebels being shot by Russian firing squad by the trackside.
This is a tragic film and, like so many late-Soviet and early post-Soviet films, very true to its times and subjects. There is no vulgar "happy ending" here: Russians would call that "poshlost", meaning something a bit tacky. The subject is tragic; the film must be also. The film is about betrayal, identity, wanting to belong, among other things.