The third episode in The Secret Service, To Catch a Spy is about a man sentenced to a long term of imprisonment for spying. He escapes and hides out with the sympathetic Sir Humphrey Burton, a man of considerable influence. But the fugitive is a threat and must be apprehended, otherwise he could cause the total collapse of the British defence system. It takes Father Unwin's sensitive approach to get into Sir Humphrey's house, and there the minimized Matthew uncovers a plot to smuggle the spy out of the country by submarine.