The game involves the intertwining stories of six characters: Sonic the Hedgehog, Miles "Tails" Prower, Knuckles the Echidna, Amy Rose (all of whom received makeovers) and two new characters; E-102 Gamma and Big the Cat. It follows Dr. Eggman's quest to once again destroy Sonic and take over the world by finding all of the Chaos Emeralds and summoning forth the ancient monster Chaos. Sonic and friends must foil his devious plan as usual.
This is the first full 3D game in the Sonic the Hedgehog franchise and was generally considered the most impressive display of graphical prowess the genre had seen at the time. Also unique is the ability to take a Chao creature found and hatched within the game and place it onto a VMU device for adventure, fighting, and breeding with other Chao on other VMU devices (they can be interconnected).
An updated version, Sonic Adventure International, was released on September 9, 1999 in North America, October 14 in Japan and Europe, October 18 in Belgium, the Netherlands, and Luxembourg, and December 3 in Australia.
This was the first game in North America and Europe where the main villain was referred to as Doctor Eggman. Previously in these locales he had been named Doctor Ivo Robotnik, with Sonic and friends using the nickname "Eggman". The manuals now refer to him as "Doctor Eggman aka Doctor Robotnik" with Sonic Adventure 2 where he starts going by the name "Doctor Eggman".
It was re-released on the Nintendo GameCube under the title Sonic Adventure DX: Director's Cut on June 17, 2003 in North America, June 19 in Japan, and June 27 in Europe, and later made available on PC CD-ROM. The update includes enhanced graphics and the playability of every Sonic the Hedgehog Game Gear game.