Grieving over the death of their son, a married couple decide to take a long yachting trip for relaxation's sake. Their journey takes a dark turn, however, when they rescue a young man from a drifting vessel. The couple soon discover that the other ship's crew had been brutally murdered by their new passenger, and find themselves in a battle of wits against this violent sociopath. Interestingly, a previous attempt had been made at adapting the novel that inspired this film by none other than Orson Welles; footage from his unfinished version, known as "The Deep," can be seen in the documentary Orson Welles: The One-Man Band.