Needing a fresh start, the Benedict family have recently moved from Chicago to a large country home on an island off the coast of Seattle. Dad Jeff (William Shatner), a dentist, has immersed himself in work, tries to stay away as much as possible because he doesn't want to deal with his wife and may have a mistress on the side. Mom Liz (Patty Duke) is unhappy, unfulfilled, feels neglected and is still fighting an alcohol addiction; which had gotten so bad before it was the catalyst for their move in the first place. Desperate for a purpose in life, she's channeled all her ill-feelings and misery over into her role as "good mother" and is smothering and overly-protective when it comes to 12-year-old daughter Tara (Quinn Cummings), who she seldom even lets out of her sight. Being prevented from playing with children in the area, Tara is lonely and bored herself and has developed a crush on an older neighborhood teen named Scotty (David Wallace). The family is vulnerable and at their breaking point, and that's the perfect time for 18-year-old orphan Joanna Redwine (Stephanie Zimbalist) to come into their lives. After all, she's a lovely, polite, well-mannered, intelligent, confident and compassionate young lady... at least at first.
After "accidentally" causing a car accident, Joanna shows up to check in on Liz and plies her with a sob story about how she'll soon be kicked out of her home by her foster parents. Liz sympathizes with her plight and offers her a summer job there doing house work and serving as a babysitter and companion to Tara. Joanna moves in and quickly proves to be an excellent cook and housekeeper, not to mention wise and mature beyond her years. She becomes a friend to Tara, encouraging her to spend more time outdoors and helping her get over her fear of water and snakes. She also becomes a confidant and shoulder to cry on mom, who is frequently upset at her husband's absence and casual neglect. As for Jeff, well he can't help but notice how attractive she is. But as things in these sort of films always do, everything eventually starts unraveling. Joanna proves herself to be a murderous psychopath with a history of tearing apart families and then murdering them before moving on to the next one and trying again.
A masterful manipulator, Joanna manages to convince Liz she can do no wrong and encourages her to start drinking again until she's a pathetic hysterical drunk. She quickly moves in on Scotty; heartlessly stealing the impressionable young Tara's crush (who will later die in a mysterious boating "accident") and dresses in mom's sexy black negligee in an attempt to seduce dad. She stops cooking and doing her housework, trashes the house during a party, turns the bitter spouses against one another and alienates the daughter from both parents. Overhearing her lying, neighboring psychiatrist Dr. Lindquist (John Houseman) immediately suspects something is off with Joanna and begins investigating matters. Though juvenile records are sealed, he is able to find the young girl's former foster mother (Virginia Kiser), who blames her for the death of her infant child. He also eventually learns Joanna has some (plastic-wrapped) skeletons in her closet... but can he reach the family before it's too late?
Director Medak had just made the excellent ghost story The Changeling (1980) with George C. Scott prior to this one and, while it's clearly not in the same league, it still isn't a bad little thriller. The entire cast does competent work and Jennifer Miller's plausible script features good characterizations and is quite sly and perceptive at times. It's also very similar to the later Poison Ivy (1992), with a charming and seductive young sociopath moving in on a vulnerable family and leaving them more screwed up than they were before she came into the picture, and also shares definite similarities to such later films as The Hand That Rocks the Cradle (1992). The intrigue unfortunately isn't sustained through to the end though and it eventually opts for a routine and predictable finale.
The Babysitter debuted on ABC in November of 1980. There was an American VHS release through HBO and it was also issued on tape in Brazil by Orion (as A Enviada do Mal), in Finland by Hollywood Home Video (as Lapsenvahti), in the UK and a few other countries. There's no DVD as of this writing, but the film has played on the MGMHD Channel.