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...