Si la police de San Francisco ne remet pas immédiatement 200 000 dollars à un homme qui vient de commettre un crime, il recommencera au rythme d'un assassinat par jour. L'inspecteur Harry Callahan est sur ses talons.
In the year 1971, San Francisco faces the terror of a maniac known as Scorpio- who snipes at innocent victims and demands ransom through notes left at the scene of the crime. Inspector Harry Callahan (known as Dirty Harry by his peers through his reputation handling of homicidal cases) is assigned to the case along with his newest partner Inspector Chico Gonzalez to track down Scorpio and stop him. Using humiliation and cat and mouse type of games against Callahan, Scorpio is put to the test with the cop with a dirty attitude.