It follows the story of Sonic chasing Dr Robotnik (or Eggman to you newer fans) across Earth in the hope to save it from the might of Chaos. This game was good because of the fact that not one single character knew the whole story, 'Why is Knuckles in the Mystic Ruins?' and 'Who is that Fat Cat?'
Once you finished the game with all six characters you got to do an extra level featuring Super Sonic, not as good as being able to do the game properly as you could in previous Mega Drive tales.
All said and done this is an enjoyable game and there are plans to make a remake onto the Nintendo Gamecube (and God forbid) PS2.