"A dead body in the library!" exclaims the librarian of the quaint village of Oberwahrdingen by Lake Constance, excitedly. Henrietta Murbel rushes in with her cape billowing, but between the bookshelves lies not a corpse, but a mannequin. Immediately, Henrietta's curiosity is piqued. Who would leave a mannequin in a library? Her detective instincts are further called into action when a real body is pulled from Lake Constance, leading the trail to the murderer in Oberwahrdingen—much to the chagrin of her nephew Tassilo, who officially leads the investigation as a criminal commissioner and has no idea how to deal with his snooping Aunt Henry.