Much of Sonic Adventure centers on Sonic, a hedgehog who has the ability to run at supersonic speeds, and Miles "Tails" Prower, a two-tailed fox who often accompanies Sonic on his adventures; they are the main characters of the game who are in pursuit of Robotnik. Supporting characters include Knuckles, an echidna who guards the Master Emerald gem on Angel Island, Amy Rose, a pink hedgehog who has a crush on Sonic, and newcomers Big the Cat and E-102 Gamma. Big is a purple, noticeably overweight cat who must rescue his best friend Froggy who became possessed by Chaos' tail, and swallows Big's "lucky charm", a Chaos Emerald, which causes him to mutate. Gamma is one among a series of robots designed by Eggman to take orders without question. After an encounter with Amy, he suffers a malfunction and gains a "conscience". He then turns his aim to destroying his robotic brothers and freeing the animals trapped inside them.

The main antagonist of the game is Dr. Robotnik, also known as Dr. Eggman because of his round body shape, who has formulated a new plan to conquer the planet, this time not relying on his robots alone, but employing a strange liquid creature known only as Chaos. Chaos also serves an antagonistic role in the game, and is the guardian of the Chao. He is apparently water or a plasma-like material and changes form after consuming a Chaos Emerald. After consuming all seven Chaos Emeralds, it turns into Perfect Chaos, which Eggman aims to use in order to conquer the planet. The main antagonist of Amy's story is E-100 Alpha (better known as Zero), the first E-100 series robot who is a large green robot and is considered the prototype.[1] Zero is one of Dr. Eggman's Robots sent to find Amy's friend Birdie. He follows Amy wherever she goes hoping to capture her and the bird. At one point he succeeds but Amy is rescued by E-102 Gamma. Eventually Zero is defeated by Amy on the Egg Carrier.

Dr. Eggman's E-Series of robots play a large role in Gamma's story, as he is one of them, and is out to destroy or "save" the others. All of them have letters of the Greek alphabet as part of their name. E-101 Beta is a black robot with two gun arms. He is the first boss in E-102 Gamma's side of the story, and is later upgraded into E-101 Beta Mk II, who is Gamma's fifth and final boss, E-103 Delta is a blue robot who is Gamma's second boss, waiting at the end of Windy Valley, E-104 Epsilon is an orange robot who is the third boss of Gamma's side of the story and is fought at the end of Red Mountain, and E-105 Zeta is a purple robot who is Gamma's fourth boss and is located at the end of Hot Shelter. While at first humanoid, Zeta is altered into a cylinder-like form with several turrets made up of a few Dreamcast machines.

Friendly characters who are non-playable include Tikal, a mysterious female echidna who appears whenever Sonic, Tails, Knuckles, Amy, E-102 Gamma and Big are sent back in time. She is trying to stop her father, Pachacamac, from stealing the Chaos Emeralds from the Master Emerald shrine. She appears mostly in the form of a red ball of light called a Hint Orb. A major feature of the game are Chao, small creatures who can be raised in the Chao Gardens and play a supporting role in the story.

Plot modifier

In the present day, Dr. Eggman learns of the legend surrounding Chaos through a mural in the lost world. Believing it to be true, he creates his flying fortress, the Egg Carrier, seeks out the Master Emerald and shatters it, freeing Chaos in the process while interrupting Knuckles's job of protecting it and forcing him to collect the shattered pieces. Eggman's goal was to control Chaos, and use its destructive powers to collect the Chaos Emeralds and conquer the entire world, turning it into Eggmanland. To help him, he has created the E-Series robots, a group of powerful robots powered by animals placed inside them and programmed to obey him. Soon after, Chaos makes his way into the Mystic Ruins where Big is sleeping. Big's friend, Froggy, ends up swallowing Big's lucky charm, the yellow Chaos Emerald, and bitting off Chaos's tail which causes him to grow his own tail. Big then begins chasing down Froggy. As Sonic returns from an adventure, he sees the police trying to attack Chaos who has arrived into Station Square. He fights and defeats him but escapes. Later at night, Eggman creates E102-Gamma, the second of his E100 robots. Eggman tests Gamma's skills by having him do target practice and battling E101-Beta in order to join the team of the Egg Carrier. The next morning, Tails is out testing a new prototype plane but malfunctions and crashes. Sonic rescues him, and Tails shows him a Chaos Emerald he found which gets stolen after a battle with Dr. Eggman upon arriving at the Mystic Ruins. Amy's story begins in mid-story of all the characters. Remembering old times with Sonic, the Egg Carrier flies over Station Square, and a bird smacks into Amy. As soon as the bird drops, a robot named ZERO chases Amy. After they learn of Eggman's plans, Sonic and his friends spring in to action to stop Eggman and they all start their own separate, but intertwined, journeys for the Chaos Emeralds.

Sonic has defeated Eggman and Chaos, Tails has stopped Eggman from bombing Station Square with a missile, Knuckles has restored the Master Emerald, Amy has rescued a small bird and his family from the clutches of Eggman and his robotic minion Zero, Big has rescued his best friend Froggy, and Gamma has rescued his robotic brothers from Eggman's clutches, destroyed by a last resort point blank blast from his elder "brother" E-101 Beta MK-II and releasing the bird during the process.

However, shortly afterward, Angel Island falls once again, and Eggman is attacked by a surviving Chaos. Meanwhile, noticing that Angel Island has fallen once again, Knuckles decides to go to Sonic with the Chaos Emeralds for advice, when suddenly he finds Eggman on the island, bitter and defeated. Knuckles asks him what happened, to which Eggman responds "This is terrible! C-Chaos is...", before being attacked once more alongside Knuckles by Chaos, who steals the latter's six Chaos Emeralds.

Elsewhere, Sonic and Tails take notice that Angel Island has fallen once again, and hurry to the scene, where they find Eggman and Knuckles lying on the ground. Knuckles then reveals to Sonic and Tails that Chaos stole and absorbed the six Chaos Emeralds he was in possession of. Eggman flies off in a hurry, going to his secret hangar to use the Egg Carrier 2. Sonic and Tails then decide that they must retrieve the final emerald before Chaos, when suddenly, Sonic is teleported to the past where he witnesses former echidna chief Pachacamac and his warriors attacking the Master Emerald shrine, with the chief's daughter Tikal pleading to them not to attack the shrine before they are all killed by Chaos, who absorbs the seven emeralds and attempts to destroy the world. Sonic then returns to Angel Island, where he and Tails return to Tails' plane the Tornado 2, which uses the seventh emerald as a power source. However, they arrive too late and Chaos retrieves the seventh emerald before they do.

Meanwhile, in Station Square, Chaos transforms into his final form, Perfect Chaos, and floods the entire city. Sonic arrives, where he witnesses Eggman trying to attack Chaos in the Egg Carrier 2. However, Chaos immediately destroys it. Tikal then appears, explaining that Chaos' heart is filled with anger and sadness and the negative energy of the chaos emeralds flows through him. Chaos drains the emeralds power completely, leaving all power of the emeralds useless. Tails, Knuckles, Amy and Big arrive with all 7 emeralds and their collective positive energies (explained in Sonic Rush) along with Sonic's good heart enables him to re-ignite the Emerald's positive powers. Sonic transforms into Super Sonic and neutralizes Perfect Chaos. Chaos is not destroyed completely, but his heart is filled with joy, happiness and forgiveness. Tikal takes Chaos back with her, but leaves the Chao to live peacefully with humans. Sonic runs off into Station Square and Angel Island is put back to its original state high in the heavens.