Dr. Jeff Benedict William Shatner and his wife Liz Benedict Patty Duke have relocated from Chicago to Seattle with their 12-year-old daughter, Tara Benedict Quinn Cummings. To help with childcare and housekeeping, Liz hires Joanna Redwine Stephanie Zimbalist as a live‑in nanny, and the family gradually settles into a new routine. Liz begins to actively engage Tara in outdoor activities, aiming to wean her off endless television, while Joanna becomes a constant, evolving presence in their home.
Doc Lindquist [John Houseman], the Benedicts’ neighbor, watches Joanna with growing wariness. One day he overhears her telling his grandson Scotty David Wallace that she had lived in Seattle before joining the Benedicts, a claim that doesn’t quite square with what Liz has been told. Doc begins a quiet, persistent probe into Joanna’s past, trying to piece together a history that seems muddled by numerous foster placements and the fact that Joanna is legally an adult. This uneasy tension underscored by Doc’s cautious inquiries casts a shadow over the otherwise peaceful household.
Liz’s mounting stress compounds the situation. She confides in Joanna about her own past year of sobriety, and Joanna suggests there is nothing wrong with having a drink now and then. Liz’s restraint falters as Joanna’s influence deepens, and a party at the Benedicts’ home loosens the boundaries that once kept the family in check. When Jeff leaves Liz at Doc’s place during the celebration, Joanna makes a provocative move, waiting for Jeff in the master bedroom. Jeff rejects the advance, but Joanna later tells Liz that Jeff had entered her room, fueling Liz’s fear of infidelity even as her drinking spirals.
Doc’s investigation intensifies as he tries to verify Joanna’s history. A clerk at the Department of Social Services can’t provide definitive information because Joanna, now an adult, is beyond the agency’s protection. Undeterred, Doc tracks down an address from Joanna’s past and visits the home of Mrs. Welford [Virginia Kiser], who fostered Joanna years earlier and warns Doc to stop pursuing her, hinting at dark, long-buried secrets. Liz dismisses Doc’s findings as gossip and refuses to involve the police, while Jeff remains torn between concern and his own complicated feelings for Joanna. Doc remains convinced that something is deeply wrong and eventually invites the family on a boat outing, hoping to observe Joanna in a less controlled setting.
Joanna’s manipulations become more overt as she grows careless with Tara and lax about household duties. During a sail with Scotty and Tara, Joanna abruptly releases the jib boom, striking Scotty and hurling him into the water unconscious. Tara pleads for help, but Joanna sails away from the danger, leaving Scotty to drift. Doc urges the police to investigate Scotty’s death, but there is no compelling evidence that it is anything other than an accident. Doc’s suspicions intensify as he eventually locates Joanna’s last address, connecting the dots between the past and the present danger.
Meanwhile, Liz, confined to bedrest, finds herself drawn into Joanna’s manipulations as Jeff finally yields to Joanna’s advances. The next day, Joanna serves Tara and Jeff raw, whole beef tongue for dinner. She then descends in Liz’s negligee and kisses Jeff, who apologizes for sleeping with her and fires Joanna, promising to take her to Seattle the next morning with some money.
Doc’s latest discovery brings the truth to a climactic moment: Joanna arrives at her last house to find the bodies of three people in the master bedroom, wrapped in plastic. The confrontation escalates as Joanna knocks Jeff out and chases Tara into the basement. Tara tries to wake a sedated Liz, while Joanna wields a kitchen knife and returns to the bedroom. Jeff wakes, wrestles the knife away, and Doc arrives with the police just as Joanna is stopped. She is escorted away in a patrol car, and in the final, haunting moment, Tara offers Joanna her doll as the family’s ordeal ends.
