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 ...