Gameplay
Sonic Adventure is a 3D platform game with action and role-playing elements. Players control one of six anthropomorphic protagonists as they venture to defeat Doctor Robotnik and his robot army, who seek the seven magical Chaos Emeralds and the evil entity Chaos. Six player characters are unlocked as the game progresses, each with their own story and attributes. Sonic the Hedgehog performs a spin dash, homing attack, and light-speed dash; Miles "Tails" Prower flies, swims, and attacks robots using his tails; Knuckles the Echidna glides, climbs walls, and punches; Amy Rose can defeat enemies using her hammer; Big the Cat is slow and carries a fishing rod he can cast; and E-102 Gamma can shoot laser beams.
At the start of the game, the player is placed in one of several Adventure Fields, open-ended hub worlds inhabited by advice-giving non-player characters. The player character is guided and instructed by the voice of Tikal the Echidna. Through exploration, the player discovers entrances to levels called Action Stages, some of which must be opened using keys hidden in the Adventure Field.:141–142 Once the player accesses an Action Stage, they are tasked with a specific objective, which is different for each character. Sonic must reach the level's end like in prior Sonic the Hedgehog games,; Tails must reach the end before Sonic; Knuckles must find three hidden shards of the Master Emerald; Amy must solve puzzles and avoid being caught by a robot; Big must fish for his pet frog; and Gamma must fight his way through stages using projectiles as a defense.:140
Some levels include minigames separate from the main story. These feature different styles of gameplay, among them rail shooting, racing, and sandboarding. Some minigames can only be accessed with particular characters. Fulfilling certain objectives allows the player to obtain bonus items. Unlocked minigames and stages the player has completed can be accessed from a Trial Mode on the title screen.
Like previous Sonic the Hedgehog games, players collect golden rings as a form of health: if the player character is in possession of rings when they are hit by an enemy or other hazard, they will survive, but their rings will scatter and blink before disappearing. Canisters containing power-ups such as speed shoes, additional rings, invincibility, and elemental shields are also hidden in levels. In several stages, the player engages Robotnik or Chaos in a boss fight and must deplete the boss's health meter to proceed. Each character starts with a limited number of lives, and the player loses a life if the character drowns, gets crushed, or is hit without any rings in their possession. The game ends when the player runs out of lives. Lives can be replenished by collecting 100 rings or a 1-up.
Players may also discover Chao Gardens, hidden, protective environments inhabited by Chao, a virtual pet. Players can hatch, name, and interact with a Chao, and raise the status of their Chao by giving it small animals found by defeating robots. The Dreamcast's handheld Visual Memory Unit (VMU) allows the player to download the minigame Chao Adventure, in which their Chao walks through a course to evolve and improve its skills. Evolving one's Chao improves its performance in competitions called Chao Races. Eggs that can produce special types of Chao are hidden throughout the Adventure Fields. Players can earn emblems by playing through Action Stages, searching through the Adventure Fields, or winning Chao Races. Each Action Stage has three emblems that can be earned by replaying the stages and fulfilling objectives, such as beating the level within a time limit.
Plot
Doctor Robotnik seeks a new way to defeat his longtime nemesis Sonic and conquer the world. During his research, he learns about an entity called Chaos—a creature that, thousands of years ago, helped to protect the Chao and the all-powerful Master Emerald, which balances the power of the seven Chaos Emeralds. When a tribe of echidnas sought to steal the power of the Emeralds, breaking the harmony they had with the Chao, Chaos retaliated by using the Emeralds' power to transform into a monstrous beast, Perfect Chaos, and wipe them out. Before it could destroy the world, Tikal, a young echidna who befriended Chaos, imprisoned it in the Master Emerald along with herself.
Seeking to use Chaos, Robotnik releases it from the Master Emerald, and tests the creature's natural form on the city of Station Square. When Sonic sees local police fail to defeat it, he and Tails work to stop Robotnik from empowering it with the Chaos Emeralds. At the same time, Knuckles, the only remaining echidna, sets out to find the shards of the Master Emerald and repair it. Activating a new series of robots, including one named Gamma, Robotnik orders them to find Froggy, an amphibian who had eaten a Chaos Emerald after mutating from contact with a piece of Chaos the night he was released. His owner Big seeks to find him as well. In Station Square, Sonic's friend Amy finds a Flicky being pursued for a Chaos Emerald in its possession, and decides to protect it. When both are captured, Amy convinces Gamma not to work for Robotnik, who helps her to escape, before seeking out and destroying the other robots in his series, sacrificing himself in the process.
Despite Sonic managing to disrupt Robotnik's plans, Chaos manages to absorb all the Chaos Emeralds. Transforming into Perfect Chaos, it rebels against Robotnik and attacks Station Square, devastating the city. Having experienced flashbacks from Tikal, who was released herself, Sonic realizes that Chaos has been in constant torment and sorrow, and that imprisoning it again will not stop it. Using the Emeralds brought to him by the others to transform into his super form, Sonic fights Perfect Chaos and defeats it. Returned to normal, Chaos calms down when he sees the Chao living peacefully in Station Square. Realizing its pain is gone, Tikal decides to take it somewhere safe to live in peace. With Sonic having won, he decides to leave to pursue a fleeing Robotnik.
