In the heart of New York City, a string of bizarre murders has left the police baffled. Each victim, a single man, is found shot to death in his apartment while the song "Sea of Love" plays on a turntable nearby. Detective Frank Keller, a weary and emotionally drained officer, is assigned to the case. Recently separated from his wife, Frank struggles with loneliness and alcoholism. Despite these challenges, he is determined to solve the case and begins to piece together that each victim had placed a personal ad in a newspaper. Teaming up with his partner, Detective Sherman Touhey, they concoct an unorthodox plan: place a similar ad and go undercover to meet the respondents at a speed-dating event.

As Frank navigates the world of lonely hearts, he meets Helen Cruger, a mysterious and alluring woman who responds to the ad. Drawn to her in spite of himself, he finds it increasingly difficult to maintain his professional detachment. Torn between his duty to solve the case and his growing attraction to Helen, Frank delves deeper into the undercurrents of love, obsession, and fear. As evidence mounts and suspicions grow, tensions rise within him as he realizes he may be getting too close to someone who could be connected to the murders. With an intricate web of desire and danger enveloping him, Frank is forced to question who can be trusted as he edges closer to uncovering the truth.