Story: Although the plot of the game differs depending on which character is taking part in the adventure, the plot mainly revolves around Dr. Robotnik's latest attempt at world domination. The creature Chaos (a new character to the Sonic games) becomes more powerful every time he absorbs a Chaos Emerald. Robotnik begins his search for the Chaos Emeralds in order to help Chaos gain his full power. It's up to Sonic and his friends to stop Robotnik's plan while learning more about Chaos and a girl echidna named Tikal.

Levels: There are two different types of stages: action and adventure. Action stages are the typical stages found in Sonic the Hedgehog games. The character starts at the beginning and works his or her way to the end of the stage. Adventure fields allow a character to explore a world and perform actions to unlock an action stage. There are three adventure fields: Station Square, Mystic Ruins, and Egg Carrier. The typical flow of the game is adventure field, action stage, and adventure field.

There are six playable characters in the game: Sonic the Hedgehog, Miles "Tails" Prower, Knuckles the Echidna, Amy Rose, Big the Cat, and E-102 Gamma. (Super Sonic becomes available as a character when the game is beaten with the previous six characters. However, Sonic does not become Super Sonic until he is ready to fight the final battle in the game.) Big and E-102 are new characters, while the physical appearance of the other characters have been redesigned.

Each character plays in different types of stages. Sonic plays in stages that revolve around running and are as follows: Emerald Coast, Windy Valley, Casinopolis, Icecap, Twinkle Park, Speed Highway, Red Mountain, Sky Deck, Lost World, and Final Egg. The stages that Tails plays are similar to Sonic's, but the stages are shorter and are a race against Sonic. The levels that Tails plays are: Windy Valley, Casinopolis, Icecap, Sky Deck, and Speed Highway. For a different type of stage, Knuckles explores various stages in search of the missing pieces of the Master Emerald. His levels are: Speed Highway, Casinopolis, Red Mountain, Lost World, and Sky Deck. Amy Rose uses her Piko Piko hammer to keep herself and a bird safe while being chased by Zero (a robot new to the Sonic games). Her levels are as follows: Twinkle Park, Hot Shelter, and Final Egg. Big uses his fishing pole to help find his friend Froggy in the following levels: Twinkle Park, Icecap, Emerald Coast, and Hot Shelter. E-102 uses a gun to rescue robots that are a part of the E-series and plays in these levels: Final Egg, Emerald Coast, Windy Valley, Red Mountain, and Hot Shelter.

There are also sub-levels/games, which include Sky Chase Act 1 and Act 2, Sand Hill, and Twinkle Circuit.

Chaos Emeralds: The seven Chaos Emeralds do appear in the game, and part of Sonic's goal is to collect the emeralds. However, the emeralds are obtained (and also stolen) as the plot progresses, rather than by playing special stages. Sonic becomes Super Sonic in the final story of the game.

Note: As previously mentioned, the Master Emerald also appears in the game.

Miscellaneous: A new part of the game introduces creatures called Chao. These lovable creatures live in Chao Gardens and can become more powerful if they are given animals that Sonic collects in action stages. The Chao can also race against each other.

Emblems are also a new part of the Sonic games. A total of 130 emblems can be earned in Sonic Adventure in Adventure Mode or in Trial Mode. (The difference is that in Trial Mode you have only four lives.) Emblems are earned in adventure fields, earning high scores on mini games, winning Chao races, and by completing certain tasks in action stages. Action stages have three types of emblems to earn: A, B, and C. The C emblem is earned for completing the stage for the first time. B emblems are earned by beating a stage with at least 50 rings (except for Big, who has to catch a 1,000 gram fish and Froggy). Finally, A emblems are earned by completing a stage within a specified time limit (except for Big, who has to catch a 2,000 gram fish and Froggy). For obtaining all 130 emblems, you get the pride that comes with mastering the game.

With many new innovations, Sonic Adventure is not easy to forget. The game was later released as Sonic Adventure DX Director's Cut for Nintendo's GameCube, with a few new additions.