Summary: A romantic cruise on a luxury yacht ends in tragedy: the hostess is murdered. When the yacht reaches shore, special detectives Duval and Dorking are asked to solve the mystery. Duval points out that because the murder was committed at sea, the guilty person is still on board. Students are given the background of each of the people involved, and a list of clues the detectives found. They are asked to break into small groups and solve the mystery.