Sonic Adventure is a 1998 platform game for Sega's Dreamcast, and the first main Sonic the Hedgehog game to feature three-dimensional (3D) gameplay. The story follows Sonic the Hedgehog, Miles "Tails" Prower, Knuckles the Echidna, Amy Rose, Big the Cat, and E-102 Gamma in their quests to collect the seven Chaos Emeralds and stop Doctor Robotnik from unleashing Chaos. Controlling one of the six characters—each with their own special abilities—players explore a series of themed levels to progress through the story. Outside the main game, players can play minigames like racing and interact with Chao, a virtual pet.