Fred Martin, an ex-con safecracker turned honest London cabbie, witnesses an armed robbery and chases the getaway car in his off-duty taxi. The thieves escape, but Martin's attempted act of heroism draws a dangerous proposal from the police. They want to"fire" Martin from his job, in the hope that the robbers, who need the services of a safecracker, will try to recruit him into their gang. When the crooks take the bait, Martin becomes a central player in a major bank heist. The sting proceeds smoothly until one of the gang recognizes Martin as the cab driver who chased them earlier, and they decide to murder him after the robbery. Unaware that his cover is blown, Martin helps the gang carry out their crime. With his police protection diverted by a falselead, Martin is left at the mercy of his ruthless new bosses. His only hope is an army of fellow cabbies who mobilize a citywide search.