A currency forger devises a very subtle method of circulating his handiwork: by blackmailing an employee of the manufacturer, he obtains duplicate keys to three safes and replaces genuine notes with forgeries. The first such swap, in Barcelona, is eventually traced to a company that had not been burgled - or so the owners thought. When forgeries then appear in Zurich, Duval spots the common factor - which leads him to the third safe, in London.