Tintin crosses the Channel bound for Sussex, then heads north to the Hebrides, in pursuit of a gang of counterfeiters, a German spy, and a mysterious "monster".

Tintin clashes with the villainous Dr. Müller for the first, but not the last time. Müller is the mastermind behind a vast European counterfeiting operation. After numerous incidents, Tintin succeeds in breaking up this criminal network.