"Prolalo Leto" ("Summer is Lost" in English) is an enjoyable Russian comedy about the summer of two 11-year-old boys. The 2 boys, Valera Bulyshev (school bicycle champion, and jack of all trades) and Eugene Ruchkin (his clumsy friend), are from Moscow. Their parents send them on a summer vacation to the small town of Kurepka, where Valery's three aunts (Dasha, Masha and Sasha) live. Valera is unhappy about the prospect of spending a boring summer in a small town with three old women and claims that summer is lost. While on the train to Kurepka, the boys meet a man that claims to be a head of an expedition, and follows the supposed adventurer onto a ship. Valery escape to the river trip. Eugene arrives in Kurepka, where the three aunts (who have never met their nephew) assume him to be their nephew Valery. Eugene is at first disappointed about staying with the aunts (one of them had just bought 40 kilograms of porridge to feed the nephew during the summer), but later finds that he enjoys himself in the small town. The aunts turn out to be fun and take him fishing, hunting and scuba-diving.
Valery (the real nephew) soon finds that his adventure is not so fun. So he goes to Kurepka, and gets into trouble there for setting dogs free and the people of the town thinks he is a hooligan. Since the aunts think Eugene is Valery and consider Valery being a hooligan, Valery hides himself in the attic of the aunts' house and has Eugene sneak food to him. The aunts become suspicious one evening when they find that their nephew took an extra 9 kotletti (hamburgers) and 13 pastries after the meal.
The adventures continue as the boys enter a bicycle race where many strange things happen. Will the identities of the two boys be finally corrected?