Trio of lazy policemen are suddenly forced to find people to arrest or else lose their jobs. At the same time a smuggling ring is operating literally on their doorstep.
Will Hay comedy (basically a remake of his earlier Oh, Mr. Porter! has fitfully funny moments, but those are largely offset by the fact that just about everyone in the film is unpleasant. And it doesn't help that there really isn't much of an ending. A shame, because there are some great lines ("Guy Fawkes wasn't a criminal--he invented fireworks") and Marriott, at least, is amusing. Hard to believe that barely a decade later studios like Ealing would be turning out masterpieces of comedy and radio program-based movies like this would become a dim memory.
Note: Desmond Llewelyn is mentioned in many sources as playing the "Headless Horseman," which he may well have done (how can you tell, though?), but he is also one of the revenue cutter sailors as well.