Shannon's annual end-of-summer pool party at her parents' isolated Lake House is always a hot ticket for the in-crowd. With her parents away, rowdy bikini-clad teenagers get wild, loud and...lewd. But little do they know, their party attracts an unseen and uninvited guest - Ron, a sadistic escaped mental patient on a murderous rampage. Seven years earlier, Ron went over the edge when he caught his girlfriend having sex at a party with his best friend. He killed them both, and has been locked away ever since...until now. One by one teens start to disappear as Ron massacres them in unspeakable ways, but the missing guests go unnoticed as everyone is having too much fun. As the party thins out, Shannon invites her three closest friends to "sleep over" and keep the party going, entirely unaware of the bloody, horrific, non-stop nightmare awaiting them.