- Colonel Lysander Stark – Head of a counterfeiting gang who's also a murderer.
- Victor Hatherley – Hydraulics engineer whose thumb gives the story its title.
- Elise – The mysterious German woman who tries to warn Hatherley of his danger.
- Mr. Ferguson – Stark's silent partner in crime.
- Inspector Bradstreet – One of Scotland Yard's crew, on the hunt for this counterfeiting gang.
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