When a psychologist is found stabbed to death in a seemingly sealed room, Inspector March must decide who had a reason to kill him, and how the person accomplished the task.

Colonel March's niece, a believer in spiritualism, thinks she hears her dead husband's voice, and others in the house become convinced her husband has returned as a ghost.

A French fashion model asks for Colonel March's help, claiming a strange man is following her and she fears for her life; several hours later, she's found murdered -- in mismatched color, a telling clue for the inspector.

Members of the Himalayan Mountaineering Club are threatened by what appears to be the abominable snowman, and someone leaves a strange footprint on a ledge outside Colonel March's office.

While on holiday in France, there is a murder at the docks and Colonel March is asked by a friend of the French police to assist; the only clue left behind by the culprit is a sailor's cap, setting off a search for the head that fits the hat.

The Second Mona Lisa, painted six months after the first, is targeted by rival collectors; Colonel March is determined to reveal the truth.