A troubled couple, Australian naval officer John Ingram (Neill) and his wife Rae (Kidman) take a break on their yacht in the Pacific after a tragic automobile accident in which their small son was killed while Rae was driving. The couple hope that this trip will help Rae cope with the loss. However, their holiday turns into a nightmare after they rescue an even more troubled stranger, Hughie Warriner (Zane), a man who claims to have lost his companions on another, now-disabled boat due to food poisoning.
Suspicious of Hughie's story, John tries to convince the panicked Hughie to return with him to his sinking ship, so they can confirm the deaths and salvage anything of use. Hughie, however, refuses to go back.
In an attempt to find answers, John travels to the sinking ship while Hughie is resting. He tells his wife (who isn't convinced that Hughie is lying) to equip the shotgun and keep it with her. Instead of equipping the gun, Rae decides to let her husband go adventuring while she plays fetch with her dog. Upon his arrival on the ship, John discovers that Hughie's original story was untrue. He finds several bodies but realizes that the dead were murdered. A tape of a film reveals that Hughie was the cameraman for a low-budget production, during which he snapped and murdered the director and four women involved. Now knowing the truth, John tries to rush back to his yacht to warn Rae.
Meanwhile on the Ingram yacht, Rae sees John across the water yelling for her to get off the boat. The sleeping Hughie awakens, breaks out of the room in which he was locked, struggles with Rae before knocking her out, and turns on the boat's motor, abandoning John and traveling away with Rae onboard. John returns in his dinghy to the sinking ship to try to catch up and save Rae.
After a slight delay spent emptying the ship of water, John is able to start the ship's motor and start his gradual chase. Rae awakens to find Hughie piloting the ship and playing with the dog. She attempts (unsuccessfully) to convince him to go back for John, but he refuses because he told John not to board the ship in the first place. She then goes down below, collapsing on her bed and weeping. Later, she heads for the radio and tries to determine the distance between their yacht and John's location. She receives Morse code contact from John using the damaged radio on the sinking ship. Rae tells John that she will find a way to return for him. They decide that Rae should stop the yacht in order to buy some time for John to catch up, and she removes the keys so that the yacht cannot travel further. Hughie soon realizes this and attempts to retrieve the keys, which she tosses into the water. Unfortunately, her dog dives into the ocean and recovers them (thinking she was playing fetch). Frightened, Rae runs into the bathroom with the angry Hughie banging the bathroom door as hard as he can. Once Hughie calms down, Rae opens the door and smiles, leading Hughie to believe that she likes him and was just playing around.
Rae, made sullen by her failed attempts, contacts John to inform him that she has failed. John signals that the ship is going to sink within six hours, and Rae promises that she will get the yacht there by sunset. Before she can tell her husband that she loves him however, John's radio breaks down completely, and they are cut off. Rae begins to cry with helplessness.
A lustful Hughie sees Rae teary eyed and frightened and begins to soothe her. While Hughie is kissing her hands and embracing her, Rae realizes that he is seducing her and wants to have sex. She acquiesces, while formulating a plan in her head about what she'll do.
They stare at each other and Hughie moves in for a kiss. Rae resists slightly at first, then quickly gives in sharing what appears to be a long, passionate kiss. After they kiss, Rae becomes excited and motionless as Hughie strokes her bottom lip and moves his hand down slowly to grope her right breast.
Hughie then leads Rae on the floor (Rae feeling coerced into this but following with growing excitement nonetheless) and they quickly start to kiss and undress on the ground. Hughie lustfully caresses Rae's figure while ripping off her shorts and underwear causing Rae to moan loudly in surprised arousal. With both completely nude, they begin foreplay with Hughie kneading her backside as Rae slowly kisses him. Rae then says that she needs to go to the bathroom. Hughie casually let's Rae go as she slides up off of him, while seductively kissing his chest, and slips on her short black robe. In actuality, she heads up on deck to put together the shotgun. The dog shows up and begins to bark for attention. Rae whispers for him to be quiet, but by then it's too late as Hughie had already heard the dog. Suspicious, Hughie gets up from the floor to look for Rae.
Knowing that she doesn't have enough time to load the gun, she quickly sees cigarettes and grabs hold of them leaving the unloaded gun behind. Hughie is relieved when she brings out the cigarettes which she says are for after they have sex. Rae then goes to kiss Hughie some more. Hughie is flattered by this and believes that she wants to continue from where they left off. To distract Hughie away from the gun though, Rae seductively leads him to her bed. In the bedroom, Rae gets naked again and sits up on her bed while Hughie stands right in front of her caressing his hand all over her body. Rae begins to quiver and close her eyes in a pleasing fashion during this. Hughie leans down to kiss her neck causing Rae to quirk her eyes and jerk her head slightly. When she opens her eyes, she stares up at his face while Hughie strokes her lips yet again and moves in to mount her.
As Hughie lays on top of Rae he starts kissing her breasts which seems to fascinate Rae as she looks on. The door opens causing Rae to jerk her head in shock (thinking that John is opening the door). Alas it was the family dog looking to to see what was going on. Feeling uncomfortable about having sex with another man in front her dog, she cleverly tells Hughie that she wants to put the dog up onto the deck (in hopes that she can also finish loading the gun). However, Hughie is so sexually aroused at this point that he tries to ignore her request and finally takes the matter into his own hands by telling the dog to go away. The dog listens and heads out the door leaving Rae behind with Hughie passionatly kissing all over her face. Feeling that there are no other options at this point, Rae firmly wraps her arms and legs around Hughie and readies herself for sex. Hughie firmly enters her which seems to cause Rae some discomfort initially but his intense lovemaking soon has Rae panting and moaning orgasmically where both she and Hughie eventually climax together in bed as the dog reenters the room.
Back on the sinking ship, the mast falls over, covering the exit hatch and trapping John. He continues driving the boat until it no longer works, and as the room floods completely, he grabs a small tube and sticks it out of the hatch, allowing him to breathe. Suddenly, he sees fish inside the ship and realizes that there must be a hole. He finds it and breaks out of the ship. He sets the sinking ship on fire to signal for help.
Meanwhile Rae and Hughie are still in the bedroom. While Hughie is getting dressed in John's clothes, Rae (back in her robe) sits on her bed feeling guilty at herself for being an unfaithful wife and enjoying sex with the man who left her husband stranded out at sea. Although it isn't made explicitly clear the shots of the changing weather conditions seem to suggest Rae and Hughie spent a few hours below deck indulging in frenzied and passionate sex. With time running out, however, she focuses on rescuing John by continuing to play the part of "Hughie's lover". She gets up and allows Hughie to play some music while they kiss and hug each other. When Hughie leaves for the bathroom, Rae fixes a drink, slips in some of her sleeping medicine, and pretends to drink it as he returns. She then offers it to Hughie, telling him to fix her another one while she dresses.
Back on deck, Rae finishes loading the gun and hides it when she hears Hughie coming. Hughie is suspicious again until Rae buys time by saying that he should ready the yacht for a coming storm. When Hughie returns, he finds her with the gun and fights to get it away from her. The fight wears down Rae, who drops the gun and hides. Hughie then feels the effects of the sedative and recklessly uses up the ammunition, which allows Rae to locate John's spear gun and lock herself in the bedroom. Once Rae locks herself in her bedroom she continues to shoot the spear gun at the pounding door. When she sees blood running from the open end of a spear in the door, she assumes that Hughie got hit. However to her surprise he comes out from behind the door to strangle her. He almost succeeds were it not from the drugged drink which causes him to finally pass out. Rae discovers that the blood was actually that of her dog whom she accidentally killed with the harpoon sticking out of his neck. After tying up Hughie, Rae gets dressed in warmer clothes and steers the boat towards John.
Later one, Hughie wakes up and cuts himself loose with a broken mirror piece that he shattered, but Rae knocks him out with the spear gun before he could do anything. She later releases a rescue raft from the yacht, pushes Hughie's body into it, and watches him float away. Rae later sights the now-blazing shipwreck in the distance and eventually finds John in a raft next to the signal fire he has set. Holding out her hand, she pulls him from the raft as the yacht speeds by.
Soon after, the couple are once again happy (Rae never revealing to John her intimate time with Hughie). They discover the floating raft with Hughie's body missing. Trying to forget and put aside what she went through with Hughie, Rae grabs a flare gun and blows out a flare to sink the floating raft leaving John to wonder what had happened when he was stranded. Later on the next day Rae is swimming in the ocean to cleanse herself as John looks down on her. Rae climbs back on the boat and sits as John soothingly washes her hair with his hands. She closes her eyes in pleasure feeling happy to be back with John. John leaves for a moment to prepare their meal. Suddenly a pair of bloody hands (obviously Hughie's) begin washing Rae's hair, she soon discovers it is not John washing her. She begins to scream for help until Hughie strangles her. John comes back looking at the struggle and quickly grabs a flare gun. With the flare gun, he fires it into Hughie's mouth and Hughie's head flares up as he falls off of the ship. Rae and John run back to each other and stare at Hughie's body float away into the ocean.