Moretti is living with her aunt and uncle in Baltimore. Her American mother has died, and her Italian father is at sea ten months of the year. Learning that Frankie has run away from two boarding schools, her father reluctantly takes her with him to live on Il Destino, the cargo ship he captains. When the two of them arrive at the ship, they find that one of the crew, the wiper, has been murdered.

Meanwhile, the wiper's killer, the Dodger, is in trouble. He killed the wiper before finding out where the wiper had hidden an important document on the ship, and his boss threatens him with death if he doesn't find it. If the Dodger can't find it on his own, he has a back-up plan: kidnap the captain's daughter and hold her hostage until the captain and his crew find it.

Frankie also has a quest: to find the wiper's killer, thereby removing what she believes is the only threat to her staying with her father on the ship. Using the methods of her two favorite detectives, Hercule Poirot and Miss Marple, she sets out to expose the killer. Her adventures begin in the port of Baltimore, continue in New Orleans, and then on the ship as it crosses the Atlantic. While Frankie tries to find out the identity of the killer, the Dodger continues to search the ship for the document. If he doesn't succeed, he will resort to his back-up plan.