Sonic and the gang go exploring another deserted 'ex-planet,' looking for emeralds.  When Cosmo and Tails enter a canyon cave, Cosmo accidentally presses a hidden button that causes them to plunge into a huge underground cavern. The two spot some weird fireflies which they follow to a huge Metarex fortress! Once inside they discover a chaos emerald factory which uses a real emerald to create fake ones. But when they’re spotted by the Metarex, they grab the emerald and run! Just when they think they're safe, the Metarex catch up with them and all seems lost. Luckily, at the same moment Sonic and the rest of the gang show up and help them defeat the Metarex and make their escape.