Donovan and Celeste Graham fly to Rome to embark on a seven day Mediterranean voyage on the Diamond Star cruise ship. Celebrating Donovan's new movie deal, this is a chance to unwind and enjoy his well-deserved sabbatical. Their tranquil shipboard experience is soon interrupted when a threatening note is delivered to their suite's mailbox. At once, Celeste realizes the note was intended for the neighboring stateroom, but is disturbed by the harshness of its message. Officer Benson, head of security on the Diamond Star, vows to take care of the situation and urges them to disregard it and take pleasure in their vacation. As directed, the Grahams relax into the romance and carefree lifestyle aboard the ultra-luxury ship, unaware that a killer with a specific target list is among the elite passengers-a killer who loathes the wealthy and plans to eliminate the next person on his list. Soon the Grahams learn that their neighbor, an illustrious business woman, Patricia Simpson, is murdered. Since embarking on the Diamond Star, Patricia Simpson's condescending and distasteful behavior offended all who came in contact with her; everyone from Donovan to the Captain. Donovan and Celeste soon are passing up the romance of the seas to help Officer Benson unravel the murder mystery and recruit the help of their retired police officer friend back home, Thomas Bell. The suspects on board appear to be justified in their loathing of Patricia Simpson and have peculiar pasts that link them to the victim. The Grahams must use their sleuthing skills in a hurry to prevent the killer from claiming another life at sea.