In the shocking third act twist of the horror movie “Cobweb,” Peter and his substitute teacher Ms. Devine manage to lock Peter’s vengeful older sister, Sarah, back in the basement pit. However, prior to this, Sarah has already committed gruesome murders, including dismembering several kids and poisoning their parents with rat poison.
The revelation that Sarah has become a literal monster comes as a surprise to Peter, who believed his parents were the real monsters for locking her away. Neglected and hidden behind the walls of the old house, Sarah manipulates Peter into believing that their parents will kill her and then lock him away as well. Fearing for his life, Peter joins forces with Sarah to stop their parents, ultimately leading to their deaths.
However, Sarah’s freedom doesn’t bring about a reunion with her brother. Instead, she plans to go on a murder spree and punish Peter for the normal life he had while she suffered in captivity. Sarah’s transformation into a feral creature is attributed to years of abandonment by her parents due to her severe deformities.
Despite Ms. Devine’s intervention, Sarah manages to lock Peter in the same pit that was once her prison. The movie ends with Peter being adopted by Ms. Devine, but the looming threat of Sarah’s escape hangs over him, suggesting that the cycle of abuse and violence initiated by their parents will continue.