The inhabitants of South Island are in danger! Dr. Robotnik has stolen all the Chaos Emeralds which kept the power balance. Robotnik has thrown the Emeralds in a parallel universe and is keeping the red one. Sonic and Tails must retreive all the Chaos Emeralds before the destruction of South Island.
This is the first Game Gear game where you can play either Sonic or Tails.
In the US version of the game, the Gigalopolis Zone was changed to the Gigapolis Zone. This could be due to the fact that dimmer children might read the name as "Gigolopolis." It's odd that the same people who'd be so anal about such a comparatively innocent association could also be the same louts who allowed Phantasy Star III's villain to be translated as "Dark Phallus."