Antonio Gomez, a nearly down-and-out musician, is a widower with a young son, Paco. Fighting to support his boy in the face of unemployment and neighbors who want custody of his son (something that here in Mexico City they might just obtain), Gomez argues with an ex-girlfriend over money she owes him. After he leaves, the girlfriend is murdered by the religious-fanatic serial killer terrorizing the city. When neighbors report the argument Gomez had with the dead girl, the police presume they are finally on the hot trail of the serial killer, and Gomez is their target. Gomez goes to a pawnshop to buy his son a long-dreamed-of guitar and meets a young woman with whom he goes looking for his son. What Gomez does not realize is that a police dragnet is closing in on him and that his boy Paco actually witnessed this most recent murder and has been trailing the killer. Before long, the killer is trailing Paco.