Allie Jones, a software designer in New York City, breaks up with her fiancé Sam after discovering his infidelity. Seeking a fresh start, she decides to find a roommate to share her spacious apartment. Enter Hedra Carlson, a seemingly shy and timid woman who responds to Allie's ad. Initially, Hedra appears to be the perfect roommate, helping Allie through her emotional turmoil and quickly becoming a close friend. However, Allie soon starts to notice unsettling behaviors from Hedra, who becomes increasingly possessive and exhibits odd quirks.

Hedra begins to adopt Allie’s mannerisms, style of dress, and even goes as far as mimicking her hairstyle. Allie’s discomfort grows as she uncovers disturbing truths about Hedra’s past and realizes that her new roommate's obsession may have dangerous consequences. The tension escalates as Hedra's true nature is revealed, leading to a suspense-filled confrontation that tests Allie's strength and resolve.