Der Andere (1959)

Italian frogman and scientist Paolo Rocello is found dead on a houseboat. The owner of the boat, Mr. Cooper, has disappeared without a trace. The English boarding school town of Medlow is now in turmoil. Of all people, the likeable boarding school teacher David Henderson is increasingly suspected of murder. This is because Katherine Walter, the niece of Dr. Sheldon, saw him on the boat on the day of the murder. However, Dr. Sheldon himself also becomes a suspect. Inspector Ford and Sergeant Broderick are in the dark – until another murder occurs and a completely new perspective on the events emerges...