Ko se Holly Parker preseli v novo stanovanje, je prepričana, da je našla popolno sostanovalko. Tess Kositch je prijazna in zadržana mladenka, ki bi bila zelo rada njena prijateljica. Toda pod Tessino sramežljivo zunanjostjo se skriva obsedena morilka, ki je najbolj srečna takrat, ko ubija prijateljice, ki so nesrečne. Holly je namreč pred kratkim izvedela, da jo je fant varal z njeno bivšo sostanovalko, zato je nadvse vesela, da pri novi prijateljici najde tolažbo. Ko postaja vse bolj očitno, da je Tess motena in obsedena s Holly, poti nazaj več ni.
In New York, the PRs colleagues and roommates Holly Parker and Jan Lambert are disputing a promotion in their agency. The unethical Jan deceives Holly and sends her to Chicago; meanwhile, she seduces Holly's boyfriend David Kray, in the opening of the fancy restaurant of his own, and they have one night stand. When Holly arrives back home, she finds that David cheated her with Jan, and she decides to move to a new apartment. She schedules a visit with the needy Tess Kositch, and they become roommates and friends. When Tess cuts and dyes her hair identical to Holly's style, she sees that her new roommate is obsessed on her. When Holly follows Tess to the underground night-club "Sin", she realizes that the girl is deranged. But Tess wants to be her friend and put Holly out misery, eliminating her former bad friends.