Detective Damian McQuaid, of New York's First Homicide Squad, decides to help a woman who has been receiving obscene phone calls. Such petty squeals are none of McQuaid's business, but when, as he leaves for the day, he overhears an attractive woman talking to a precinct detective, he volunteers to help her. Unofficially, of course: he knows just what interpretation his buddies in homicide would put on that if they find out.

Because she is terrified of being alone, he spends the night on Iris's sofa, but when he wakes the next morning, he finds that she has been murdered in her bedroom while he slept.

Shocked and angry, he decides to investigate on his own time—again, very unofficially; again, for very obvious reasons—and his inquiries lead him to begin a clever, insidious, and dangerous program of harassment in an effort to expose the man McQuaid believes to be the killer. All very unofficially, because McQuaid doesn't have a single clue that would convict his suspect.