The film depicts a Soviet family of the professional class. The head of the family, Fyodor Ivanovich, is a doctor and lives with his son, Boris, and daughter, Irina. His mother and nephew Mark also live with the family. The film centers on Boris's girlfriend, Veronika, and her experiences in wartime. She is nicknamed "Belka" ("squirrel").
The call to war sounds, and the country responds with great patriotic fervor. Boris volunteers to defend his homeland from the attackers, much to Veronika's sadness. He leaves for the war, and is killed under the birches (pod beryozonykoy) in a swamp, saving a fellow soldier. However, he is listed as missing in action and neither Veronika nor his family know that he has been killed.
The German blitzkrieg then begins, and the family is terrorized by nights of bombing; they take refuge in the subway system underneath the city. During one attack, Veronika leaves her parents in their apartment and when she returns the building had been bombed. The entire apartment is gone and there is no sign of her parents, so Fyodor Ivanovich invites Veronika to live with his family. Mark, who has been pursuing Veronika for much of the film, is put in charge of keeping her company to lift her spirits. He is obviously in love with her, but she continually rejects him because she is waiting for Boris to return. However, during one bombing raid Veronika refuses to go down to the shelter and is left alone with Mark. This scene is unclear, but it appears that Mark rapes Veronika. She is then shamed into marrying him, and the rest of the family believes that she has betrayed Boris.
The family is relocated with many other Russians further East to escape the German offensive. They live in a temporary community where Fyodor Ivanovich, Irina, and Veronika work in a military hospital. Mark spends his time partying and playing music; he and Veronika are both clearly unhappy in their marriage. When one of the soldiers in the hospital becomes hysterical when he receives a letter saying that his girlfriend left him, Fyodor Ivanovich gives a speech condemning her. Veronika overhears the whole thing and becomes very upset, since the situation seems to parallel her life.
Shortly after that, Fyodor Ivanovich finds out that Mark was deferred from conscription not because he was considered too talented to be exempt as he had claimed, but because he bribed an official in Fyodor Ivanovich's name. Fyodor Ivanovich suddenly realizes that Mark has not only betrayed Russia, but he has betrayed the whole family and has taken advantage of Veronika. Fyodor Ivanovich confronts Mark and kicks him out of the house, while Veronika is allowed to stay and is forgiven by the family for "betraying" Boris.
The man that Boris died trying to save comes in search of Boris's family to tell them the news. When Veronika finds out, she refuses to believe it, saying Boris's friend Stepan, who volunteered with Boris, will know what happened to him. When the soldiers come back at a victory parade Veronika meets Stepan and she finds out that Boris is indeed dead. The film ends on a hopeful note when the war is over: Boris's friend Stepan makes a speech asserting that they will never forget those who died in the war, but peace must be maintained.