The six Find-Outers are having a chance tea at a local gymkhana with Inspector Jenks and his goddaughter Hilary when a robbery occurs in a nearby large house. The mysterious robber disappears from the scene of the crime without a trace – as if he were invisible and cannot be found. The robbed house turns out to be that of Hilary, so the children have the perfect excuse to investigate as they take the upset girl home.

          The mysterious thief leaves only a few clues behind – enormous footprints, enormous glove prints, a strange criss-cross mark on the ground, and two torn pieces of paper. The clues do not seem to make any sense. Of all the Peterswood villagers, only policeman Mr Goon and Colonel Cross have feet big enough to fit the footprints, and the thief cannot be either of them. The Five Find-Outers and dog decide that they will find the culprit before Mr. Goon does.

          Fatty uses his disguises to gather important information, and in doing so outwits Mr Goon, especially when both go at the same time to see Colonel Cross to ask him about his large shoes. Mr Goon disguises himself three times but on each occasion the Find-Outers see through the disguise straight away.

          The thief strikes again several times, once in Fatty’s own shed,and on each occasion the same clues are found – but apparently nobody sees the thief.