Eggman can triumph over Sonic in this iteration of Sonic Unleashed by using the immense power that the Chaos Emeralds provide to capture Sonic. The overwhelming might of the assault had not only shattered the planet’s surface into innumerable fragments but also freed the shadowy and mysterious being known as Dark Gaia from inside its depths. Eggman then abandons Sonic and the now-empty Chaos Emeralds on the shattered planet and departs. Sonic is left behind with the broken earth. It is now up to Sonic to rescue the world, reclaim Dark Gaia, and finally defeat Eggman once and for all to restore the Chaos Emeralds after they have been depleted.
The game has both daytime and nighttime stages, with the former concentrating on Sonic’s overdrive speed and the latter emphasizing battle. During the nighttime levels, Sonic transforms into a terrifying-looking but strong Werehog beast as a side effect of his attack from the Chaos Emerald.
The PlayStation 2 and Wii game versions use a different gameplay engine and have a level selection almost entirely different from the original.