On the first of September, Vitya Maleev arrives in fourth grade. Having never had time in arithmetic, Vitya forgot even the multiplication table over the summer. His teacher Olga Nikolaevna believes that Vitya is a capable boy, only lazy.

Vitya decides to take up his mind, but after school he wants to play soccer with friends. When Vitya sits down for lessons, he first makes the easiest, and he has no strength left for arithmetic. Parents are trying to explain how to solve problems, but tired Victor does not understand.

A new student Kostya Shishkin comes into the classroom, with whom Viti makes friends. Kostya's father died at the front, and the boy lives with his mother and her sister. Kostya, who is fond of animals, gives Vita two white mice, but Vitya’s younger sister, third-grader Lika, takes care of them. Kostya writes off his homework from Vitya, and Vitya decides to follow his example.

Vitya begins to receive deuces. They are being worked out with Bones at a meeting. Wounded Vitya decides to observe the regime of the day in order to work out his will and pull himself up by his studies, but he cannot overcome himself.

Somehow, due to bad weather, Victor cannot go for a walk. He does his homework, but decides not to do arithmetic, but ask his classmate for help. But he, an avid chess player, suggests playing chess.Carried away by chess, Victor studies them according to the textbook and begins to beat his friend.

The class is preparing for the evening of amateur performances. Neither Kostya nor Vita Olga Nikolaevna permits to participate - first you need to fix the deuces. With the help of Lika, friends make the frame of the horse, which will be depicted on stage in the production of Ruslan and Lyudmila. Because of the performance and because of chess, Victor does not have time to catch up on arithmetic and gets a deuce in a quarter.

Shamed by parents, classmates and Olga Nikolaevna, Victor sits down for arithmetic. Thanks to the help of a classmate, he begins to succeed. Lika asks him to help her solve the problem. In order not to drop his authority in front of his younger sister, Victor sends her out for a walk, and, having gathered, he solves the problem. Taking a textbook from Lika, Vitya stubbornly solves problems from there. Now it becomes easier for him to solve problems from his textbook, and he pulls himself up in arithmetic.

Kostya, unlike Viti, throws his studies. Somehow, to avoid a deuce on the control, Kostya pretends to be sick and does not go to school. The mother, having guessed, threatens to expel the recently brought homeless puppy Lobzik and other animals. Kostya gives the animals to friends, and Lobzika sets aside, promising his mother that he will correct the marks.

After going with the class to the circus, Kostya decides to train Lobzik and become a circus artist. Kostya skips classes and trains Lobzik, deciding that he does not need to finish school to work in the circus. Vitya is forced to lie to Olga Nikolaevna that Kostya is sick.

Jigsaw does not give in to training, and Kostya decides to become an acrobat.Vitya comes to a friend every day and does homework with him, so Kostin’s mother doesn’t know anything.

Vitya is ashamed of the fact that he covers Kostya, but then classmates decide to visit the "patient", and the fraud opens. Olga Nikolaevna promises to ask the director to let Kostya continue to study and talk with the mother of the truant.

The director is talking with both friends. He believes that Victor should help his comrade catch up on material. Learning about the training of Lobzik, the director advises how best to do this, and offers to speak at the school evening.

Kostya pulls himself up to study. In costumes tailored by Lika, the guys successfully perform at the school New Year's Eve.

As corrected deuces, Vite and Costa offer to do public work. The guys create a cool library. Kostya becomes neat and collected. And in the fifth grade friends go with one of the fives.