10-year-old Rosmarin von Stetten lives with his widowed father Robert in the big city. He is the leader of a gang in school called "Hallodribums" (spell: hullowdreebooms). The boys decide to make a lot of mischief during their vacation. When Rosmarin comes home, he hears that he has to go to the castle of the "von Stettens" alone - and is disappointed. He explains that he wanted an adventurous vacation, but his dad tells him that he should go, but that his mischief should be so that it would be a good deed too, and so he agrees. Aunt Charlotte awaits him with her daughter Beate. The following day in the morning Rosmarin goes to the lake near the castle and meets Thymian, the poorest boy of the village, whose mother did not come back from the war and who has to do several jobs in the village for his living. The boys enter a boat, but both fall into the lake. Thymian crawls out of the water first and is found by a servant from the castle, who thinks he is Rosmarin and brings him to the castle and to bed. The boy doesn't know what happens and thinks he's spellbound. In the meantime Rosmarin is found by some village people and brought to Thymian's hut. Then he is ordered to help in the store. When he learns that Thymian is in the castle, he knows he has to do a good deed and to work for him, so that Thymian can enjoy the good life on the castle. But soon he recognizes that the work isn't as easy as he thought. In school he surprises the teacher with his knowledge of geography and French. But then a message arrives that his (Thymian's) mother will come home, finally, honored for her samaritan work in the war. He tries to change his role with Thymian, but the mother is back too soon - and he has a wonderful day with her. In this moment Robert von Stetten comes to the castle and sees the boy. But soon he hears that it is the son of Anna, the girl he wanted to marry years ago. But his relatives were opposing. At last at a village festivity the boys can change their personality, and because Robert and Anna see the similarity of their sons, they stay together - all four.