Sonic Unleashed, known as Sonic World Adventure in Japan, is a Sonic platformer released on across several platforms. The games are essentially the same across all platforms, although due to the lower capabilities of the PS2 and Wii, these versions of the game have some areas and elements removed or simplified.
The game introduces a new character to the series, Chip, and has two different gameplay elements controlled by the games day and night system. Day time stages are played as Sonic with traditional Sonic platforming. The night time stages are played as Sonic in a Werehog form, which have a slower, more action-orientated element.
The story begins with Sonic and Dr. Eggman in battle aboard his spaceship, after transforming into Super Sonic, Eggman is able to make a grab for him and drains him of his chaos energy. With the absorbed energy, Eggman releases a Dark Gaia, a monster that was in the centre of the planet and, in the process, splits the worlds continents apart from the planets surface. Eggman then ejects Sonic into space.
When Sonic returns back on the Earth he encounters a friendly creature who is suffering from amnesia whom Sonic decides to name Chip. Sonic also realises that the ray used by Eggman to extract the chaos energy from his body has had the unusual effect of causing him to transform into a Werehog at night. Sonic and Chip then set off to repair the planet, restore Chip's memory and cure Sonic of his werehog ability.
Music from the game features in the following CD soundtracks: