The Stepdaughter

Jesse Conner runs a ranch in California with his ailing wife Maggie, his daughter Linda, and his son Buddy, who goes to a special education school. When Will, one of their horse handlers, is murdered at a hotel in Los Angeles, Jesse hires Susan Heller to fill Will's position. Linda, who has a large collection of mystery novels, is immediately suspicious of Susan, so Linda decides to get Jesse's mechanic, Paris, to help her figure out whether or not Susan is up to something, but Jesse tells Linda that she's been reading too many mystery novels, and that she needs to lighten up. Maggie has an accident in which she falls off of a ladder and is taken to a hospital where she is treated and released, and as Maggie is released from the hospital, nurse Callista Davis recognizes Susan and calls her Karen. This arouses Linda's suspicions even more. Jesse later explains to Susan that he once bought Maggie a horse named Big Palomino, and Maggie fell off of the horse one day and needed hip replacement surgery, and Maggie has had to take pain medication ever since. Linda calls Callista, who comes over to the ranch while no one is around, and Susan uses a big knife to kill Callista, and then she leaves Callista's body in a lake. Linda, who is not aware of Callista's murder, asks local deputy Tom Gitlin to check Susan out, but he finds nothing. On Susan's birthday, while everyone is away, Susan confronts Maggie alone while Maggie is taking a bath, and Susan admits to Maggie that she is Maggie's daughter, and Susan's real name is Karen Mills. When Karen was born, Maggie's parents forced Maggie to give Karen up for adoption. For her entire life, Karen has been hurt by the belief that she was never wanted. Karen spent time in institutions and foster homes, living a life of misery, and now she wants revenge on Maggie for giving her up to that kind of life, so she kills Maggie by dropping a radio into the bathtub water, electrocuting Maggie. Linda figures out that Callista once worked as a night nurse at the Carson Correctional Facility in Los Angeles, and Karen was one of Callista's patients. Linda later steals a file from the human resources cabinet, and figures out that Karen was at Carson for a few years because she burned down a house, killing a foster father who was abusing her, but Linda is unaware that Karen killed Will so Karen could take over his position at the ranch in order to get close to Maggie. At the ranch, Karen insults Buddy, who is on the phone with Linda, and Buddy runs away from Karen. Karen heads to a nearby cave that Buddy likes to hang out in, and Paris, who wants to sleep with Karen, confronts her. Karen sees Buddy run out of the cave, and then Karen sends Paris falling to his death, down a drop-off in the cave. Karen leaves the cave to find Buddy. A few minutes later, Linda arrives at the ranch to help Buddy try to fight Karen off, but Karen may be too much for them to handle.