When a teen befriends an elderly resident at a nursing home and moves her in with her family, she discovers her new acquaintance isn't the sweet, helpless lady she appeared to be.

17-year-old Sarah Richards decides to volunteer at a local nursing home for school credit. She quickly meets one of the adoring residents, a fairly spry Mabel Braxton who is in the home recovering from an injury. Due to her energy and sweet personality, she shines among those around her like a star. Sarah takes a liking to Mabel and the feelings become mutual.

Her dismissal from the home imminent, Mabel informs Sarah that she has no place to go. Her only son has nothing to do with her. Feeling that she must help this sweet lady, Sarah convinces her parents, Ellen and Nathan, to fix up the bonus room above their garage and let Mabel pay rent to live there. The offer excites Mabel until she sees how small the apartment is. Still, she hides her disdain as she attempts to bond with Sarah and her family, including Sarah's younger sisters Casey and Lisa. Mabel even babysits the younger girls when Sarah is at the home over the summer.

Sarah becomes busy with her position at the home and bonds with Lucy, another patient who has had an exciting life. Sarah loves hearing about her exotic adventures, only it comes at a cost - not spending the amount of time with Mabel that the older woman expects. Mabel becomes jealous and sneaks into the home one night, murdering Lucy in her sleep.

Instead of Sarah spending more time with Mabel, she hangs out with her boyfriend Nick and best friend Kelli. As Mabel grows more desperate for Sarah's attention, she poisons Nick to remove him from Sarah's life. Mabel isn't finished. She volunteers at the home so she can spend time with Sarah, who hates the idea. When a patient tries to warn Sarah about Mabel, he ends up dead the very next day.

The last straw for Sarah comes when Mabel steals some of Lucy's jewelry that she left to Sarah and pawns it. Nathan and Ellen tell Mabel she has to leave, not knowing they have lit a stick of dynamite beneath the unstable women. Mabel moves out but has a master plan of getting revenge on the family she feels has done her wrong.

Ellen and Nathan go away for an anniversary weekend, leaving Sarah to look after her sisters. They have no idea that Mabel has planted listening devices in the house and hears that Sarah will pick her sisters up after school on Friday. Sarah discovers a flat tire when she leaves school and enlists help to change it. Meanwhile, Mabel picks up Casey and Lisa, and takes them to her son's vacation cabin, where she sedates the girls as she waits for Sarah to come.