Sonic Adventure is a game in the Sonic the Hedgehog series. It is for the Dreamcast and GameCube. It was the first 3D Sonic game. It was called Sonic RPG when it was being made.
A long time ago, a race called the Chao were befriended by Tikal, the chief of the Echidna tribe's daughter. However, the chief attacks the Chao to steal their Chaos Emeralds. The leader of the Chao, Chaos, became a watery monster and destroyed all of the Echidnas, except for Chaos. Tikal sealed herself and Chaos into the Master Emerald, and this caused the shrine to become Angel Island.
Sonic Adventure is divided into seven plot segments. They are Sonic the Hedgehog, Miles 'Tails' Prower, Knuckles the Echidna, Amy Rose, Big the Cat, E-123 Gamma, and Super Sonic. All of them except for Super Sonic are dedicated to the character they are named after. Super Sonic is dedicated to all the characters, but Sonic is the playable character in this plot segment. Sonic is the fastest character, and has the most stages. His levels revolve around getting to the goal to either rescue animals or get a Chaos Emerald. Tails's levels are the most similar to Sonic's. He has to race either Sonic or Dr. Eggman to the goal. Knuckles has to find three Master Emerald pieces. Amy Rose has to get to the end of the stage while running away from a killer robot. Big the Cat has to find his pet, Froggy, and fish for him to catch him. He can also catch other fish for extra points. E-123 Gamma has to get to the goal under a time limit. Super Sonic is just the final boss, Perfect Chaos.