Charlotte has an unsavory past, but she's on the straight and narrow these days. Her ex-husband Sam is now missing, but before he disappeared, he left urgent, cryptic messages about someone threatening him--someone who has been threatening Charlotte, too. So Charlotte takes a job as a personal assistant for an engagement party aboard a private luxury cruise ship, the Cleobella, to get far away from anyone who means her harm. But as the Cleobella sails through its glittering destinations, increasingly sinister events haunt the guests, and the turquoise waves and sun-drenched beaches give way to something darker.