Two youths, thinking a fruiterer keeps a lot of cash at home, break in while he is at work. His wife is tied up and gagged while they ransack the house. Finding nothing, they decide to question her again, but when her gag is removed she screams for help. The youths panic, and in their attempt to silence her they bash her to death. They flee empty handed just as the fruiterer, Sergio Conte, returns. He recognises one of them as a customer at his shop, and he swears to avenge her death and sets out to catch the Killers before the police. This leads to a race between detectives and Sergio to find the killers, and results in a dramatic chase through an underground tunnel system.