A British diplomat, looking natty in a green hat, walks into the brooding hills above Lake Como and disappears. The police investigate but can uncover no clues or evidence. This is a job for British Intelligence in the person of none other than the intrepid Tommy Hambledon.

Hambledon takes on Italy's vicious underworld, is diverted by some society types, and side-steps some unrepentant fascists in this rapid-transit chase for a mastermind and a hidden treasure.