May is in full swing and life is in full swing at the dachas near Moscow. While their parents are busy, Varya and Seryozha go for a walk with their funny Scotch terrier Stepashka. On the river they rescue a strange cub. The baby is unusually cute and does not look like any animal known to them. The children decide to take him home and name him Lucky. Secretly from the parents, they begin to nurse the baby, which costs them a lot of effort and ingenuity. However, they fail to maintain Lucky’s presence in the house for long. Adults try to find out who Lucky is, and come to the conclusion that this is an animal unknown to science. After much discussion, they decide to keep him in the house. There is plenty to worry about with the growing Lucky, but despite all the difficulties, an upbeat atmosphere reigns at the dacha. Dad jokes tirelessly, comes up with competitions, puts on a play, celebrates non-existent holidays, tells fascinating stories and organizes pranks, invariably getting into various comical situations along the way.