Harold Petts (Spike Milligan) is a conscientious village postman who receives a promotion that takes him to London, to be trained at London's busiest post office. However, after his first day in the big city he is soon in trouble. In the main sorting office he succeeds in beating the new mail sorting machine at pigeonholing letters for delivery (the machine blows up in the process). As a result he is placed safely out of the way in the parcels department, but sorts parcels at such speed that he puts everyone else in the department out of work. This leads him to a meeting with the staff psychiatrist, and then to Jean (Barbara Shelley), an ambitious Art school, and the pair find themselves the main suspects in a mail theft ring, with the police and post office officials hot on their heels.