While his parents are on vacation, twelve-year-old Victor Laroche wants to uncover the mystery surrounding the death of a girl who fell over a banister in the same house 20 years earlier. His sisters Cora and Louise help him. Through stories told by an elderly uncle, the father of the girl named Cäcilie, they come across a former friend of Cäcilie's, Friedrich Mörlin. The series title refers to a small leather crocodile that Victor finds in the house during his investigations.