A colorful cast of characters is on board a luxury cruise line from the United Kingdom to Italy. One of which is a wealthy Baroness, who is looking for an escape after receiving a series of threatening postcards marked with ABC. She hires a private detective, who poses as her butler to avoid suspicion during the cruise.

On the first night, she receives a message from ABC containing one of her most precious gems, warning her that he had been in her cabin. This leads the Butler/Detective to suggest she confine herself to her cabin until he can find her stalker.

In the morning, he finds her dead body. Her tea has been poisoned and all of her jewels stolen! The Butler/Detective assembles everyone on board to the dining room.