A San Francisco city bus, with eight passengers and the driver, pulls out of a downtown bus station and moves through the city stopping once for a new passenger. The passenger, unseen above the chest, walks to the back of the bus pulls the pieces of a sub-machine gun from a tote bag, assembles them, and massacres the eight passenger and the driver. The bus crashes and the killer walks away. Driving onto the scene are homicide detectives Jake Martin (Walter Matthau), Leo Larsen (Bruce Dern) and James Larrinore (Lou Gosset). As they search the bus they find one of the bodies is that of Dave Evans (Anthony Costello), Martin's police partner. It is the search for the murderer and the reason for Evans' presence on the bus that pairs detective Martin and Larsen together. With the help of Evan's girlfriend Kay Butler (Cathy Lee Crosby), they determine that Evans was following leads to close a murder case which Martin, sixteen years on the force, was unable to solve two years previous. Thed search takes the two detectives to most of the seamy sides of San Francisco, with its hustlers, porno theatres, topless and gay bars, boarding houses and restaurants, from the rankest to the plushest. Aware that he has been forced upon Martin, that Larsen, a veteran detective in his early 30's , tough, mischievous, blunt and slightly crass, does his best to make his replacement of Evans as subtle as possible. Martin, withdrawing into himself, ignores Larsen. In the pursuit of Evans' leads, Mratin discovers a bizarre possible solution linking the unsolved murder and the bus massacre. Unable, by the lack of evidence to explain his theory to Larsen, he attempts to convince him to pursue his theory and deviate from the the instructions of their superiors. At first, Larsen refuses, but as his respect for Martim and his unorthodox police procedure increases, Larsen agrees and the two policemen set out to trap the mass murderer.