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The movie tells the story of Billy Lenz, a young boy who was abused by his mother as a child. His mother killed his father one Christmas with a claw hammer right before Billy's eyes and kept Billy in the attic, while she started a "new" family with her lover. One night, Billy was raped by his mother, and she became pregnant with a daughter by him. After 15 years, Billy escaped the attic and murdered his mother and her lover while his sister/daughter, Agnes, looked on.

Cut to present day, a group of eight sorority sisters - Kelli (Katie Cassidy), Melissa (Michelle Trachtenberg), Lauren (Crystal Lowe), Heather (Mary Elizabeth Winstead), Dana (Lacey Chabert), Megan (Jessica Harmon), Clair (Leela Savasta), Eve (Kathleen Kole) and their house mother, Ms. Mac (Andrea Martin), who now live in Billy's childhood home, find themselves being harassed by threatening mystery phone calls during Christmas break.

The plot follows the Slasher genre, as each character is brutally murdered and has a bag put over their head by the unseen killer, until just Kelli and Leigh (Clair's sister) remain. When they think they have escaped, Kelli finds the killer, revealed to be Billy's sister Agnes, making a Christmas tree out of Kelli's friend's heads. Meanwhile, Billy returns and in the ensuing fight, Billy and Agnes are apparently killed while Kelli is rushed to the hospital with Leigh who suffered minor injuries