Children's detective stories were very successful in the 1960s, and the stories of Martin the criminal even had several sequels. In this film, Martin investigates a mysterious ancient crime in a castle where today the guide mentions this fact only as a curiosity. He is led to this by a broken display case on which he finds a child's handprints. And in a seemingly mundane red slide, easily mistaken for a rare diamond, he finds an important clue...

It is the summer holiday and little Mísa Horák has taken a few animals from the school's so-called "Corner of Living Nature" home to care for. But when he brings home two Angora rabbits, a starling, a little monkey, a tortoise and a small crocodile, his parents order him to take them away. He is helped by Káta, who offers to place the animals in the child's room of her family's apartment. Mísa leaves for her friend where the animals will hopefully be able to stay. Káta goes shopping. In the meantime, Káta's younger sister Minka wakes up and begins to play with the animals, who gradually run off in disarray.