Tintin follows Müller and Ivan to Kiltoch. In Kiltoch he gets to know about Black Island where everyone forbids him to go because they think there is a beast there. However, this does not keep Tintin away. He discovers that the Black Island is the forgerers' headquarters. He also discovers that the ""Beast"" is a huge ape called Ranko. On Tintin's notification, the police are able to apprehend the entire gang.

Tintin and Haddock set out to rescue Professor Calculus.