Jean hops on a plane to Hawaii, planning to meet her friends Michelle and Mandy. Originally, she planned on leaving when they did, but had to stay an extra day to makeup a test she missed. She meets Mike, a good-looking guy from a small town that won a free trip to Hawaii. They talk for awhile and he talks about a mysterious place in Hawaii. She thinks it’s odd that he knows the place, given that he has never been there before.
She doesn’t have a chance to talk to him about it though because he goes into convulsions. The flight attendants work on him, but he dies. Jean cries in her seat and eventually falls asleep. She talks to one flight attendant before climbing off the plane and they both mention how nice Mike was.
Mandy picks her up from the airport and tells her all about the things they already did. Jean is pissed because she cannot afford to do everything and has to use coupons. They do whatever they want, including renting a more expensive car and doing a bunch of stuff without her. They also met scuba instructors Johnny and Dave. Michelle hooked up with Dave and Mandy claims Johnny for herself.
Mandy asks her not to flirt with him when they get introduced. Jean asks him a few random questions and Mandy accuses her of going after her man. She also meets Dave, who seems more concerned with hooking up than anything else. Both men act weird when she points out that she might not want to go scuba diving.
The girls take a scuba lesson and both Jean and Michelle are naturals. Mandy keeps messing up, including shooting to the top of the water in a panic, which would kill her in the real ocean. Dave demands that Mandy stay after for another lesson. Michelle tells Jean that the week should be interesting because Johnny has eyes for Jean, but Mandy is still throwing herself at him. The boys teach them about air embolisms and when she asks if they ever saw someone die from one, Johnny hesitates and Dave jumps in with a firm, “no.”
The boys take the girls on a private dive and once again, Mandy keeps screwing up. Jean asks a lot of questions and learns that they had another partner. They all go out for dinner and Johnny lets it slip that their partner died. Michelle asks about treasure hunting and Dave tells them the story of the Moonflower, a ship that crashed the year before. Their partner tried looking for the treasure, but never found it.
Mandy goes home, leaving Jean with Johnny. They take a walk on the beach and he tells her that Michelle took scuba lessons when she came the year before, even though she told Jean she never took a lesson. He gets a call that his mom hurt herself and he tells Jean that she is an alcoholic. Jean meets her and notices that she looks perfectly fine, no scratches. Johnny downplays the event and suggests she come out on the boat with them the next day. She turns him down because Mandy can’t go.
Jean also has two dreams about Mike. In one, she sees him in the bathtub bleeding. When she wakes up, she finds the bathtub full of red liquid, but realizes that it is Michelle’s ketchup bottle from lunch. She has another dream where she sees Mike blowing up a balloon until it explodes. She wakes up as Dave grabs her and pulls her back. She started sleepwalking and almost walked off the balcony of their hotel room.
Dave lets all three girls go out on the boat, but complains about them taking up space. Jean convinces Johnny to take the waverunners and go to a remote spot because it reminds her of her dreams. Mandy tags along, has a problem and Johnny calms her. Jean ventures into an underwater cave and sees a skull, but Johnny drags her out.
Both guys tell her that she was seeing things due to oxygen deprivation. Dave dives into the cave and comes back, saying he didn’t find anything. She gets Johnny alone and tells him her suspicions. She tells him about her dreams and how she felt compelled to dive in that exact spot. Jean calls Patricia, the flight attendant and talks to her about Mike. Patricia tells her that Mike died a month before on a dive and that she slept through most of the flight.
Jean drags Johnny to the newspaper office and finds the article about Mike’s death. The police found him off a small island and ruled his death an accident caused by his lungs exploding. They thought he came up too fast, but it was odd because no boat was nearby and they had no idea how he got there. Jean rushes to the cemetery because apparently he was apparently never shipped home. She finds his grave and asks him for help, right before a tree crashes down and knocks her out.
She dreams of standing with Mike and he shows her what happened. He dived with someone else, found a skeleton and the other diver killed him by unplugging his air. When she wakes up, she rushes back to the hotel and sees the police standing around. Someone fell from a high balcony. Jean talks to one of the cops and tells him that she thinks it was one of her friends.
He will not let her see the body, but eventually does and she sees Mandy. He listens to her, but when she starts talking about bad dreams and he sees her head wound, he doubts her story. She rushes off and asks Johnny to meet her back near the underwater cave.
Jean steals a jet ski, goes down and finds the skull. She comes up, swims to shore and finds Johnny. She warns him about Dave and rushes off. She comes across both Dave and Michelle knocked out and with nasty head wounds. Johnny takes her to safety, but she notices his red wetsuit and remembers that the man who killed Mike wore a red suit. Johnny realizes that she knows the truth and holds her captive on the boat.
Johnny tells her that his old partner Ringo found the big safe from the shipwreck. They went down to get it and he knew Ringo was going to steal it, so he killed him and left him in the cave. The safe sank, so he convinced Mike to go out with him for a moonlit dive. When Mike found Ringo, Johnny killed him. They fight and Jean manages to jump overboard. He tries to kill her, but she gets to his air tank first and he dies.
Jean swims back to shore and finds Dave. She tells him what happened and they help Michelle. She has a dream about Mike thanking her for solving his death. When she wakes up, she is sitting beside his grave. A voice calls to her and she sees Michelle and Dave coming towards her. Jean stops, sees necklace she gave to Mandy on Mike’s tombstone and silently thanks him before walking towards her friends.