Tintin, Captain Haddock, Calculus, the Thompsons and Wolff uncover the wonders of the moon. They also discover another person aboard the rocket ship who threatens to leave them all stranded on the moon.
Tintin and Captain Haddock have been asked to be part of the expedition to the moon. As they help Professor Calculus prepare for the launch, Tintin persists in finding the internal spy.
After receiving a telegram from Professor Calculus, Tintin and Captain Haddock travel to meet him at the atomic centre in Syldavia. They soon discover that the secret rocket ship documents have been stolen.