San Francisco Police Department Inspector Harry Callahan's testimony against crime kingpin Lou Janero puts the mobster in prison. Callahan becomes famous and the target of Janero's men, both of which he dislikes. After Callahan kills four attackers during an ambush, the department assigns Al Quan as his partner; Callahan advises him to get a bulletproof vest, as his partners often get killed. They investigate the heroin overdose death of rock singer Johnny Squares, found in his trailer during filming of a slasher film directed by Peter Swan at the Port of San Francisco. We see that his death was not a typical overdose, but is actually murder.