Johnjo O'Shea is an ordinary 17-year-old, with the exception that he is a hemophiliac. He is asked to give a few friends of his older brother a lift in his brother's car, so that they can visit a pizza takeaway. Johnjo is unaware that one of his passengers has been tipped off by a friend who runs that shift at the pizza place that an adversary is about to enter, and the real reason for wanting a lift is to get there quickly to confront him. One of the other boys starts an argument with a bystander (Tommy Ward), and stabs him, fatally. The drama then follows the police and legal procedures, as well as the conflicts between family members, the boys involved and the community. Although he and his family consider him to be an innocent bystander, he ends up in court charged as an accessory to murder, under the doctrine of common purpose, the "common" of the film's title, which may also make reference to the view that segments of society might take of the working class youths involved.