Plot[ edit ] Catherine is waiting for her boyfriend to arrive at her home, where she is cooking dinner. She discovers her boyfriend's car in her driveway, and inside finds him with his throat slit. A killer wearing a skull mask breaks into the home and murders her, throwing her body in a swimming pool. Shortly after, a group of international students from an elite Prague school are graduating; among them are the American Sarah; the Irish Mike; the English Frank; Kim, an Australian; Carmen, from Germany; Diego, from Argentina; and others.
To celebrate, Sarah's boyfriend Greg plans to break into an indoor swimming complex with the help of Martin, an American former student and employee of the water park, giving the friends free access to pools and water slides. On finals day, Kim is upset over failing her tests. She and Mike, her boyfriend, go swimming at a lake, and she tells him she feels like an outcast among him and his wealthy peers; she decides not to attend the party.
Later, Kim is accosted in the woods by the masked killer, and stabbed to death with a machete. Late that night, the group arrive at the water park for the party, and are let inside by Martin, where they begin drinking and swimming in the pools.
Svenja and Carter go to an isolated pool to ride a water slide. While Carter waits for her at the bottom, his is stabbed to death. As she descends the waterslide, the killer drives a machete through the bottom of the slide; she attempts to stop but is unable to, and the machete slices through her groin. Carmen and Chris discover their bodies floating in the small pool.
They locate the remainder of the group, but are unable to leave the building are as the entryways have been locked. Meanwhile, Martin and Mel find an empty weight room to have sex. When Martin goes to retrieve a condom , Mel is attacked by the killer chased into a locker room where she is killed. Aware that a killer is loose, Sarah, Mike, Carmen, and Chris find Martin en route back to the weight room. In a panic, they all rush to save Mel, but find her corpse in a bathroom stall.
The group are unable to locate Greg, Frank, or Diego, and Martin finds that the lock picks and spare keys to the building have been removed; the phones have also been disconnected.
Martin recalls an air duct leading out of the building, and proposes the group exit that way. He goes ahead to dismantle a vent grate, after which Sarah, Mike, and Carmen follow; Chris stays behind, as they are unable to turn around and lift him from inside the duct.
While they crawl along, the killer begins driving his machete through the bottom of the duct, killing Mike and Martin. Sarah and Carmen escape through a different duct and find themselves in the basement, where they are confronted by the killer, who chases them through the building.
They momentarily hide in a security monitor room and ponder who the killer is; Carmen believes it is Greg. Meanwhile, Diego and Greg come across Chris, terrified.
Greg decides to enter the air duct to find a way out, and Diego waits with Chris. Greg finds Sarah and Carmen in a room overlooking the pool. Convinced he is the killer, Carmen knocks him unconscious, and she and Sarah lock him inside. A trained swimmer, Carmen attempts to swim into a filter chamber to exit the building. She injures her leg, but is able to access the exterior of the building. Outside, she finds a security guard's corpse with a gun lying next to it, but before she can retrieve it, she is stabbed by the killer and thrown back into the filter chamber.
Inside, Sarah is confronted by Frank, who reveals himself to be the killer, motivated out of spite and jealousy over his failed romantic advances on each of the women. Sarah manages to light him on fire, but he leaps into the pool. Greg breaks free of the room overlooking the pool, and also jumps into the water, and the men begin to fight.
Sarah enters the water and manages to stab Frank with a broken bottle. He rises from the water and Greg and Sarah attempt to flee, but is shot to death by Carmen, who survived the earlier attack.