Official story and features:
"The original "Sonic Adventure" returns with enhanced graphics and more than five hours of new gameplay, including an all-new Mission Mode featuring more than 50 missions and hidden surprises for gamers to discover.
It's up to Sonic and his pals to save the world from the evil Dr. Eggman, who has returned to once again try his hand at world domination. The evil Doctor's scheme is to steal the seven Chaos Emeralds and raise the all-powerful Chaos monster to do his bidding. Entangled in the plot of the sinister Dr. Eggman, Sonic must enlist the help of his friends Amy, Tails, Knuckles and Big to foil Eggman's every move."
Story from Sega site
"- New graphically-enhanced 3D world harnesses the power of the Nintendo GameCube
- Six playable characters each with his/her own skills and abilities
- Thirty massive levels, over-the-top boss battles and mini-games featuring the one-of-a-kind Chao AI characters
- "Sonic Adventure DX Director's Cut" connects to "Sonic Pinball Party", "Sonic Advance" and "Sonic Advance 2" via the Nintendo GameCube-Game Boy Advance cable for endless on-the-go Chao raising fun
- All new Mission Mode with 60 mini-missions, allowing gamers to discover exclusive items and unlockable secrets, including 12 hidden classic SEGA Game Gear titles"
Features from Sega site