Early on an unseasonably warm Christmas Eve, Arthur Bryant of the Met's Peculiar Crimes Unit is summoned by the Home Office to attend a crime scene. Later that morning, he meets his colleague John May at a bus stop near Marble Arch. At Bryant's insistence, the two elderly detectives board an open-top tourist bus where he explains that they are in pursuit of the individual who strangled a 54 year-old cleaning lady in her flat the night before. As the old Routemaster trundles past some of London's iconic tourist sights - Oxford Circus, Regent Street, Nelson's Column, Whitehall, the palace of Westminster and even New Scotland Yard (a journey during which Arthur Bryant succeeds in upsetting both his fellow passengers and the tour guide) it becomes clear why the two policemen should have been called upon to investigate such a 'normal' murder. Because, of course, nothing is ever quite that straightforward when Bryant and May are on the case . . .