An excursion boat on Lake Constance. Good-humored passengers celebrate at a costume party. In the middle of it all is Elena Barin on the day before she takes up her post as head of department at the Constance criminal investigation department. She is traveling as a tourist - until the moment she notices a weapon on a passenger.

A man and his ship are sunk in Lake Constance and the dead man's son is missing. For the detectives, he is the prime suspect. Until an anonymous tip-off about a prisoner hidden in a box turns up...