Colonel Alexandrov has been at the front for three months now. He sends a telegram to his daughters in Moscow, invites them to spend the rest of the summer in the country.
The eldest, eighteen-year-old Olga, goes there with things, leaving the thirteen-year-old Zhenya to clean up the apartment. Olga studies as an engineer, makes music, sings, she is a strict, serious girl. At the cottage, Olga meets a young engineer Georgy Garayev. She is waiting for Zhenya until late, but her sister is still gone.
And Zhenya at that time, having arrived in a summer cottage, looking for mail to send a telegram to his father, accidentally enters someone's empty cottage, and the dog does not let her back. Zhenya falls asleep. Having woken up in the morning, he sees that there is no dog, and nearby is an encouraging note from an unknown Timur. Having discovered a fake revolver, Eugene plays with him. A blank shot that broke a mirror scares her, she runs, forgetting the key to the Moscow apartment and the telegram in the house. Eugene comes to her sister and already anticipates her anger, but suddenly some girl brings her the key and the receipt from the telegram sent with a note from Timur himself.
Eugene climbs into an old barn, standing in the back of the garden. There she finds the helm and begins to twist it. And from the helm are rope wires. Eugene, without knowing it, gives signals to someone! The barn is filled with many boys. They want to beat Zhenya, unceremoniously invading their headquarters. But the commander stops them. This is the same Timur (he is the nephew of George Garaev). He invites Zhenya to stay and listen to what the guys are doing. It turns out that they help people, especially the families of soldiers of the Red Army take care of them. But they do all this secretly from adults. The boys decide to "do it special" Mishka Kvakin and his gang, which climbs other people's gardens and steals apples.
Olga thinks that Timur is a bully, and forbids Zhenya to be with him. Zhenya cannot explain anything: this would mean divulging the secret.
In the early morning, the guys from Timur’s team fill the barrel of an old thrush woman with water. Then they put firewood for another old woman, the grandmother of the brisk girl Nyurka, at a woodpile, find her a missing goat. And Zhenya plays with the little daughter of Lieutenant Pavlov, who was recently killed at the border.
Timurovites make an ultimatum to Mishka Kvakin. They order him to come along with an assistant, a figure, and bring a list of members of the gang. Geika and Kolya Kolokolchikov carry an ultimatum. And when they come for an answer, the Kwakins lock them in the old chapel.
Georgy Garayev rides Olga on a motorcycle. He, like Olga, is engaged in singing: he plays in the opera of the old partisan. His "severe and terrible" make-up scares anyone you want, and the prankster Georgy often uses this (he owned a fake revolver).
Timurovites manage to free Geik and Kolya and lock the Figure instead. They ambush the Quakino gang, close everyone in the booth on the market square and hang a poster on the booth that the “captives” are apple thieves.
There is a noisy holiday in the park. George was asked to sing. Olga agreed to accompany him on the accordion. After the performance, Olga encounters Timur and Zhenya walking in the park. The angry older sister accuses Timur of turning Zhenya against her, she is angry with George too: why didn’t he admit earlier that Timur was his nephew? George, in turn, forbids Timur to communicate with Zhenya.
Olga, to teach Zhenya a lesson, leaves for Moscow. There she receives a telegram: her father will be in Moscow at night. He comes for only three hours to see his daughters.
And a friend comes to Zhenya at the dacha - the widow of Lieutenant Pavlov. She urgently needs to go to Moscow to meet her mother, and she leaves her little daughter for the night with Zhenya. The girl falls asleep, and Zhenya leaves to play volleyball. In the meantime, telegrams come from my father and from Olga. Eugene notices telegrams only late in the evening. But she has no one to leave the girl, and the last train has already left. Then Eugene sends a signal to Timur and tells him about his misfortune. Timur instructs Kolya Kolokolchikov to guard the sleeping girl - for this you have to tell everything to Kolya’s grandfather. He approves the actions of the boys. Timur himself takes Zhenya on a motorcycle to the city (there is no one to ask permission from, uncle in Moscow).
Father is upset that he was never able to see Zhenya. And when the time is nearing three, Zhenya and Timur suddenly appear. Minutes fly fast - Colonel Alexandrov needs to go to the front.
George does not find a nephew or a motorcycle in the country and decides to send Timur home to his mother, but then Timur comes along with Zhenya and Olga. They explain everything.
George receives a summons. In the form of a captain of tank troops, he comes to Olga to say goodbye. Eugene transmits a “common call sign”, all the boys from the Timurov team are running away. All together go to see George off. Olga plays the accordion. George is leaving. Olga says to Timur who is sad, “You always thought about people, and they will repay you the same.”
