This Cobweb article contains spoilers.
By the time poor little Peter (Woody Norman) and his caring substitute teacher Ms. Devine (Charisma Carpenter) manage to lock Sarah back in the basement pit in the depths of the old house, Peter’s vengeful older sister has already dismembered a whole bunch of kids and painted the walls red with their blood. And just before that, she convinced Peter to spike their parents’ dinner with a healthy portion of rat poison, their final agonizing moments spent vomiting the black sludge from their exploding organs all over the kitchen and foyer. Orphaned but now free of his abusive parents who lock him in the dark basement when he “misbehaves,” Peter rushes to free his long-lost sister, only to discover that she’s become a literal monster.
It’s the shocking third act twist that turns this very grim horror movie on its head. Until Sarah climbs out from behind the grandfather clock and reveals her monstrous form, Peter believed that his parents were keeping his sister locked away simply because they were themselves monsters. At least, that’s what Sarah whispers to him from the other side of his bedroom wall. Sarah has been forced to live behind the walls of this old house for years, neglected by her parents and kept hidden from Peter. It’s easy to see why Peter is so susceptible to Sarah’s influence, especially after he himself experiences some of the same abuse at the hands of Carol (Lizzy Caplan) and Mark (Antony Starr), and later finds the remains of another missing little girl in their backyard. When Sarah tells Peter that their parents will soon kill her and then lock him away to begin the cycle all over again, Peter believes he needs to stop Carol and Mark to protect himself and his sister.
Now their parents are dead too, but Sarah isn’t exactly in the mood for a warm embrace with her brother. Peter doesn’t heed her mother’s final warning — “Don’t let her out” — and is about to pay the price. Free from her captivity, Sarah plans to not only go on a murder spree but also punish her little brother for the normal life he got to “enjoy” with their parents while she was trapped and uncared for in the depths of the house. It’s here that we learn Sarah’s origin story and how she became the long-limbed, sharp-toothed, wall-climbing monster now hunting Peter. Born severely deformed, Sarah’s appearance frightened Carol and Mark so much that they decided to hide her away rather than to love her as their child, and it’s these years of abandonment that turn Sarah into a feral creature who can effortlessly behead teenagers with a single swipe of her claws.