When a gang of London thieves, disguised as policemen, begin robbing other thieves....well, that's just not cricket. Benevolent burglar Peter Sellers, the man in charge of all "respectable" crooks in town (he even offers such incentives as a vacation plan and filmed training sessions!), sets about to ascertain how the renegade criminals have received inside information concerning upcoming robberies. He arranges a temporary truce with Scotland Yard so that both criminal and constable can work together in nabbing the miscreants. Alas, he must now contend with incompetent peacekeeper Lionel Jeffries, who poses an even greater threat than the "mole" who's been tipping off the phony cops (who is closer to Sellers than he'd ever suspect). Short, simple and sweet, the black-and-white Wrong Arm of the Law manages to pack more solid laughs than any three of Sellers' later overproduced Technicolor vehicles combined.
~ Hal Erickson, All Movie Guide