Scandal breaks in the London press: there is a mole in the Intelligence Service, and it appears without a doubt, on a photo taken by MI5 agents, that the mole wears the face of Francis Blake.

Mortimer is determined to believe that his friend has been forced to act against his will, but the initial investigation sweeps this hypothesis away. With MI5 agents planning to try Blake for high treason, and to kill him if needs be, Mortimer decides to find his friend before they do.