On a quest for world domination Sonic nemesis Dr. Eggman has managed to steal the chaos emeralds once again and this time drained them of their power. As a result not only has the Earth split into distinct sections, so has Sonic. Now he is two beings. During the day he is the usual speed-addicted hedgehog players will remember, but at night he becomes Sonic the Werehog. In either form it is the player's ultimate goal to find the missing chaos emeralds and return them to their proper place in order to set the world and Sonic himself, back the way he should be.