In The Giant Diamond Robbery, Geronimo Stilton is reluctantly roped in by his grandfather Wiiliam to be his caddie for one of the biggest golf tournaments in Mouse Island. There, Geronimo meets his friend Kornelius Von Kickpaw, who is special agent OOK.
It has been discovered that there is a plot to steal the diamond-encrusted gold cup, which is the trophy of the tournament. Now, Geronimo Stilton and Kornelius Von Kickpaw have to team up and stop the thieves before it is too late. Meanwhile, the tournament itself seems to be jinxed, with golf balls going crazy and people acting abnormally.