In this game, Sonic the Hedgehog, Princess Sally Acorn and Miles "Tails" Prower investigate Dr. Eggman's Incredible Interstellar Amusement Park, where Dr. Eggman admits to the public that he has turned over a new leaf and has created an amusement park to make up for his past deeds. The park is revealed to be a cover for Eggman, who is capturing nearby planets and kidnapping the natives: an alien race known as the Wisps. With the power of the Wisps, Sonic visits each of the planets to stop the doctor's plan.
After supposedly developing a sense of remorse for his past misdeeds, Dr. Eggman opens an amusement park called Dr. Eggman's Amazing Interstellar Amusement Park, which is located in space made up of several planet-sized attractions, in order to make up for his transgressions. Suspicious of the doctor's intentions, Sonic, Sally and Tails investigate in the amusement park before the opening day. As they comment on how pretty the place is, they see Orbot and Cubot chasing two alien-like creatures; Sonic promptly rescues them, and ends up discovering wonderful powers with some of them. The aliens are called Wisps, and one of them that Sonic saved is named Yacker. In order to communicate with Yacker, Sally and Tails builds a translator in his Miles Electric. Through communicating with Yacker (albeit with complications due to the fact that the translator has several glitches that scramble the words), they learn that the other Wisps have been kidnapped by Dr. Eggman, who plans to harness their energy, called Hyper-Go-On, for his nefarious schemes.
Allying with the Wisps and using their powers, Sonic visits multiple planets, liberating the Wisps and shutting down the generators that link them to the amusement park. Eventually, Sonic soon learns that Dr. Eggman is transforming the Wisps into Nega-Wisps and using them as fuel for a mind-control cannon to make Earth the star attraction of his theme park. When Dr. Eggman tries to fire the cannon at the world, a piece of wreckage, created when Sonic destroyed the first boss, causes it to malfunction and explode.
As the amusement park begins to explode, Sonic and Sally sends Tails back down the space elevator while Sonic faces Dr. Eggman who is using a powerful robot that harnesses the negative energy of the Wisps. With the help of every type of Wisp, Sonic defeats Eggman with the Final Color Blaster. The malfunctioning cannon creates a black hole that consumes the amusement park and the defeated Eggman. While even Sonic is unable to outrun the black hole, all of the Wisps use their energy to neutralize the black hole and save Sonic as he lies fainted. Yacker changes the Nega Wisps back to normal. Sonic wakes up back on Earth, breathing heavily and looking tired. Sally and Tails reprimands Sonic for pushing him into the space elevator but thanked him for handling Eggman all by himself. Yacker arrives to thank them before returning to his planet along with the Wisps.
The Nintendo DS version of the game has the same scenario, but the game is shorter. In the DS version, Tails' translator works perfectly. After each world is beaten, Sonic unlocks missions for that planet and his friends appear in it (none of his friends, aside from Tails, appeared in the Wii version). Thus, after all of the Chaos Emeralds are collected from Special Stages, there is a special boss called Nega-Mother Wisp. She is actually Yacker's mother, who is possessed by the Negative Hyper-go-on energy. Sonic has no choice but to fight her in the form of Super Sonic.
After fighting her, she is able to turn back to her normal form, Mother Wisp. Sonic found out that she is not only Yacker's mother, but is the mother of all Wisps and the creator of Planet Wisp. Eggman captureed the Mother Wisp before the cutscene "Hyper-go-ons Charging," at the end of the Aquarium Park level, as it can be assumed the "giant alien" was her. However, Eggman does lose control of the Mother Wisp before Sonic clears the third mission in Asteroid Coaster, but he had already turned her into a Nega-Wisp by then.
- Sonic the Hedgehog
- Virtual Hedgehog (exclusively for co-op) (Wii version)
- Mii (exclusively for co-op) (Wii version)
- Miles "Tails" Prower
- Knuckles the Echidna
- Princess Sally Acorn
- Cream the Rabbit
- Amy Rose
- Big the Cat
- Shadow the Hedgehog
- Rouge the Bat
- E-123 Omega
- Espio the Chameleon
- Charmy Bee
- Vector the Crocodile
- Blaze the Cat
- Silver the Hedgehog
- Mother Wips
- Dr. Eggman
- Tropical Resort
- Sweet Mountain
- Starlight Carnival
- Planet Wisp
- Aquarium Park
- Asteroid Coaster
- Terminal Velocity
- Game Land
Note: Game Land's levels act as Special Stages. If a set of three Acts are completed, the player gets a Chaos Emerald at the end of the third. As with most Sonic games, collecting all 7 Chaos Emeralds awards the player with the ability to transform into Super Sonic, although it has to be enabled using the Options Menu.
All the Areas above (besides Game Land), once selected, will take the player to the stage-specific Area Map. Each Area Map has 6 Acts and a boss selectable, and 7 "S" ranks and 30 Special Rings to be earned. However, Terminal Velocity being the final Area has only two Acts selectable plus the final boss, with no Special Rings to collect.
Voice Reel Demos
- Roger Craig Smith as Sonic the Hedgehog
- Kate Higgins as Miles "Tails" Prower
- Travis Willingham as Knuckles the Echidna
- Kath Soucie / Tara Strong as Princess Sally Acorn
- Michelle Ruff as Cream the Rabbit
- Cindy Robinson as Amy Rose
- Kyle Hebert as Big the Cat
- Kirk Thornton as Shadow the Hedgehog and Orbot
- Karen Strassman as Rouge the Bat
- Vic Mignogna as E-123 Omega
- Troy Baker as Espio the Chameleon
- Colleen Villard as Charmy Bee
- Keith Silverstein as Vector the Crocodile
- Laura Bailey as Blaze the Cat
- Quinton Flynn as Silver the Hedgehog
- Utako Yoshino as Yacker
- Wally Wingert as Cubot
- Mike Pollock as Dr. Eggman