British teenager Leo Colston spends a summer in the countryside, where he develops a crush on the beautiful young aristocrat Marian. Eager to impress her, Leo becomes the "go-between" for Marian, delivering secret romantic letters to Ted Burgess, a handsome neighboring farmer.
The crooks in London know how it works. No one carries guns and no one resists the police. Then a new gang appears that go one better. They dress as police and steal from the crooks. This upsets the natural order of the police/criminal relationship and the police and the crooks join forces to catch the IPOs (Impersonating Police Officers), including an armoured car robbery in which the police must help the gangs to set a trap.
