When Countess Clotilde Sanfelice falls to her death from a window of her house in front of two horrified witnesses, a homeless woman who is seen fleeing the house is suspected of having murdered her. Rex and Inspectors Fabbri and Morini investigate the suspicious death.

Katia has won a cruise for two and she is accompanied by Inspector Lorenzo Fabbri, who leaves Rex in Morini's care. But Rex manages to sneak aboard, and Fabbri hides him in his cabin. Meanwhile Katia discovers that Anna, who is presumed to have been physically abused by a passenger, has disappeared.