Sonic the Hedgehog struggles to undo the damage done to the world and himself after Dr. Eggman unleashes an ancient evil which causes him to transform into a vicious Werehog by night.

The version of Sonic Unleashed developed by Dimps for the Wii and PlayStation 2 differs somewhat in design and and mechanics from the one made by Sonic Team for other platforms, including a different engine and fewer high-speed daytime levels.