Tadeusz Rosołek, a pawnshop’s employee, is enchanted by Kazia Wolska, one of the clients who, due to financial problems, pawns her watch. The woman who lives together with her mother in a large house. She asks him for help in finding school-age boys for a possible homestay. When none of the candidates finds acceptance, Rosołek disguises himself as a boy and, as his younger brother Wacuś, moves into the room to be closer to his beloved. From then on, he leads a double life. He hides his bike in the bushes in the garden, clothes and his school bag in a dog's shack, and enters and exits through the window.

From the beginning, he is jealous of Roman, whom he takes for being Kazia’s fiancée. After the supposed "school", he helps her beloved, who is a dressmaker, to deliver ready-made dresses. One of her clients, Centkowska, a rich widow who adores children, is delighted with him. After the incident with Kuba, a housemate with whom he plays dice, Kazia invites Tadeusz home and asks him to forbid his brother to gamble. The man performs miracles of dexterity, so the hoax would not be revealed.

Mrs. Centkowska, with the help of her admirer, Doctor Łęcki, brings toys for Wacusia. The “boy” complains about his brother that he had to hide in a chimney, because of fear of him. An upset widow wants to adopt Wacuś. He sends Łęcki to the pawnshop to arrange a meeting with his brother. As a result of a misunderstanding, four pawnshop employees come instead of Rosołek to meet with Centkowska.

There is a football match at school on Sunday. Kazia and her cousin Roman come to support Wacusi. The "boy", so that the lie does not come out, gets rid of the goalkeeper and takes his place. One day Kazia walks Wacuś to school. Not wanting to give himself away, the boy enters the first school they come across. It turns out to be a beauty school. When Kazia realizes that there has been a mistake, he leads Wacuś to the appropriate male junior high school. Students recognize him as the goalkeeper who lost the match. They run at him with fists. A math teacher rescues him from the oppression, but he gives him a difficult task to figure out. Wacuś does not even answer, as he is in a hurry to get to the pawnshop on time. He does not have time to change clothes. Customers do not want to be served by a boy in a youthful uniform. Grumpy and gutted, Wacuś goes to a restaurant, where he orders vodka, which he is served despite his young age, because he can pay for himself. To the joy of all the guests, he sings cheerful songs. Kazia and Roman, who had noticed Wacuś entering the restaurant, interrupt the fun. They take him home.

When Kazia explains him that Roman is her cousin, Wacuś cannot hide his joy. Kuba finds his roommate's suit and school bag in the garden. Wacuś admits that he truly is Tadeusz and confesses his love to Kazia.

• Film programmes

Program do filmu „Wacuś"

Program do filmu „Wacuś"

Program do filmu „Wacuś"

Program do filmu „Wacuś"