Every year, on average 23 people disappear without a trace from cruise ships, presumed suicides or tragic accidents.

No one has ever come back.

Until now.

Five years ago police psychologist Martin Schwartz lost his wife and son. They were holidaying on a cruise ship when they simply vanished. A lacklustre investigation was unable to shed any light on what happened – murder-suicide was the coroner's verdict. It is a verdict that has haunted Martin ever since, blighting his life.

But now he has been contacted by an elderly woman, a writer, who claims to have information regarding their fate and wants him to come on board the Sultan of the Seas immediately.

She explains that his wife and son are not the only mother-and-child pair to have disappeared. Only a few months ago another mother and daughter also vanished. It appears there may be a serial killer on board.

But when the missing daughter reappears – carrying Martin's son's beloved teddy bear – it becomes apparent that the truth could be much, much worse...