Likeable country postman Harold Petts gets transferred from his village to London, where on his arrival he unwittingly foils a mail train robbery. Innocent in the ways of the big city, he is thought to be a member of another gang by both the train robbers and the police, who all suspect him of trying to rob the post office where he works. Petts however gains notoriety in the post office by his ability to outperform the new mechanization which the sorting office has recently installed. Harold becomes a hero when he thwarts the robbers once more when they attempt to steal a mail-bag containing used bank notes which are being returned for destruction. As his reward, Harold gets promoted back to his home village as Postmaster.