A passenger is found shot dead in his seat on a London bus when it reaches its terminus. Apart from the driver and conductor, there have only been two other passengers on the bus, a white-haired man and a masculine-looking woman, who both alighted separately at earlier stops. To the investigating police, the conductor is the obvious suspect, and he is held and charged. The man's fiancee hires private detective Paul Rivington to prove his innocence - and it turns out to be his most extraordinary and dangerous case to date...