Is it this one? Sorry I had to Google it:
[The Relationship & Motive
• The Affair: Billy and Sidney dated at Woodsboro High School. Billy later discovered that Sidney's mother, Maureen Prescott, had an affair with Billy's father, Hank Loomis.
• The Abandonment: The affair caused Billy's mother to abandon the family, driving Billy into a murderous rampage that he modeled after horror films.
• The Climax: Billy and his accomplice, Stu Macher, reveal themselves to Sidney in the third act, blaming Maureen's affair and his mother's abandonment as their primary motive before Sidney fights back and kills them.]
