Patty Palmer (Melanie Griffith) and Drake Goodman (Matthew Modine) are living out their dream of buying and renovating a large Victorian home in San Francisco. Carter Hayes (Michael Keaton) appears to be the perfect tenant for the downstairs apartment. However, soon they find he is not at all what he seems — he may even be dangerous. When Patty and Drake try to evict him, Carter begins to wage an escalating psychological war on the defenseless couple — one which they can’t afford to lose. If Carter wins, it may cost Patty and Drake their relationship, their home, perhaps even their lives.
Police Lieutenant Kinderman (George C. Scott) begins to notice eerie similarities between the murder investigation he's currently working on and the techniques utilized by the infamous "Gemini Killer" (Brad Dourif) who was executed fifteen years prior. A hospitalized mental patient comes forward claiming to be the "Gemini Killer" but he resembles a priest Kinderman knew who passed away during an exorcism. As more casualties amount, Kinderman begins to investigate the connections between the two allegedly deceased.
Elliot Mantle (Jeremy Irons) is a successful gynecologist who works alongside his identical twin, Beverly Mantle (Jeremy Irons). Elliot is notorious for developing intimate relationships with his patients and giving the women to Beverly when he loses interest. Beverly develops strong feelings for a patient, Claire Niveau (Genevieve Bujold). But when she accidentally deceives him, Beverly spirals out of control, slipping into a state of madness.