This game is the first in Sonic series to feature free-roaming 3D gameplay. The game is divided into three types of stages: Action Stages, Adventure Fields and Subgames. The division of Action Stages and Adventure Fields was a serious departure from previous Sonic games. In addition, unlike the previous game in the series, Sonic Adventure has six different characters to choose from: Sonic, Tails, Knuckles, Amy, Big and E-102 Gamma; each with their own style of gameplay. Only Sonic is playable at first, but other characters are made available after encountering them in his story. When all stories have been completed, a final story is unlocked.