In 1974, at a drive‑in theater, Todd and Terry, identical twin brothers, witness their mother Maddy, Louise Lasser, kissing her date in the back of a car. Terrified by what he sees, Terry wakes his brother and the two slip away. In a burst of rage, Terry murders a teenager who is having sex in the backseat, then frames Todd by smearing blood on him and placing the hatchet in Todd’s hand. Todd, too traumatized to defend himself, is found guilty and committed to an asylum.
Ten years later, in 1984, the now‑adult Terry lives with his mother in a sprawling, secluded complex called Shadow Woods. His long‑dormant murderous rage resurfaces when Maddy becomes engaged to her fiancé Brad, who owns the property. Terry also learns that Todd, the twin he framed, has escaped from the mental hospital and may be coming home. In a brutal blaze of violence, Terry murders Brad by severing his right hand with a machete and then splitting his head, hoping to frame Todd once again.
Dr. Berman and her assistant Jackie arrive at Shadow Woods in search of Todd. Terry greets Jackie and then murders him with the machete, followed by a grisly half‑slicing of Dr. Berman in the woods. He discards his bloody T‑shirt and changes into a vest, then visits his neighbor Andrea, who is babysitting. Andrea tries to seduce Terry, but he remains uninterested. Andrea’s mother Julie, Jayne Bentzen, and her date Bill, Ed French, arrive home. Meanwhile, Karen, Terry’s friend, bumps into Todd, who has arrived at Shadow Woods, and she initially believes he is Terry. When Todd reveals his true identity, Karen flees to tell her friends, while Terry disappears into the night in search of his twin.
As word spreads, Maddy begins to panic and drink heavily. Todd comes across Dr. Berman’s body, is overcome with emotion, and takes Dr. Berman’s gun to chase his mass murderer brother. Back at Andrea’s house, Bill is decapitated by Terry, who then stabs Julie to death. Terry spies on Gregg and Andrea playing tennis, and later murders both of them by the swimming pool. Artie discovers the bodies, is held at gunpoint by Todd who tries to convince him that it is not Todd but Terry committing the killings, and then flees. Artie is later stabbed in the neck with a carving fork.
Karen uncovers the truth about the two brothers and heads to Julie’s house, where she finds Julie dead and takes the baby to safety. A highly intoxicated Maddy contacts the police and discovers Brad’s bloodied corpse. At the pool, Terry corners Karen with the baby; Todd arrives and fights his brother inside the pool as Karen pulls Todd out. Maddy shoots Terry, believing she has killed Todd, only to realize she has killed Terry. Distraught, she drags herself away as Karen flees with the baby.
In the final moments, a hysterical Maddy shoots herself in the head. Police sirens wail in the distance, leaving Todd’s eventual fate open to interpretation. The film lingers on shadowed corridors and a haunting sense that the cycle of violence may not be over, even as the night ends in tragedy.
