Sans Mobile Apparent plot
"One of them is the next victim. One is the killer. We challenge you to guess who."
Commissioner Carella is assigned to a murder case that seems to lack any motive. Then two more people are killed in the same way. The only lead for Carella is a mysterious diary that he gets from the stepdaughter of the first victim. The diary contains a list of his mistresses. One of them is also an ex-mistress of Carella. He finds out that she knew all three victims, but before he finds out more, she too is murdered. Ultimately, the stepdaughter's boyfriend leads Carella to the surprising, shocking denouement of this mystery.