It is said that a very long time ago, the people of West Side Island used the Chaos Emeralds to advance their civilization and achieve prosperity. However, when those people tried to use the Emeralds for the wrong reasons, their prosperity vanished overnight. It is believed that the gods sealed away the Emeralds within the island's depths.
Sonic the Hedgehog 2
In the present day, Sonic the Hedgehog was looking for new adventures following the events of his last adventure when he discovered West Side Island. Sonic landed his personal plane, the Tornado, on the island to check it out and relax, not paying attention to the fact that a certain metallic flying object was chasing him and suspiciously landed on the other side of the island.
One day shortly afterward, Sonic noticed a shy fox cub tagging behind him, who nervously scurried back into the welcoming shade of the still palm trees when he was spotted. When Sonic was not looking, the peculiar fox barely managed to keep up with the blue hero. The fox's name was Miles Prower, though he was known as "Tails" due to his two special tails, which often made him the target of bullying. However, something changed in Tails when he saw Sonic come to the island, and he made up his mind that he wanted to be cool like Sonic. Realizing that the young kid was harmless and could actually keep up with him, Sonic allowed Tails to hang around as he freely explored the island.
On an early afternoon, Tails discovered a lone aircraft stranded at the beach. Being a budding fan of machinery and vehicles, he rushed to take a closer inspection. He was admiring its magnificent build until he noticed that Sonic was taking a nap under its wing. Realizing that Sonic owned the plane, Tails gently retreated to the other side of it to avoid waking him. Suddenly, an ominous flash occurred in the direction of the nearby forest, followed by a terrific explosion. The wild fire blew away various debris, which Tails avoided by taking cover underneath the Tornado as he witnessed robots violently excavating the area. This ceaseless activity threatened to blanket the entire island in its destructive flames.
It turned out that Dr. Robotnik had detected the presence of all Chaos Emeralds after following Sonic, including the lost seventh one spoken of in the island's legend. He took the opportunity to kidnap the island's Animals to rebuild his Badnik forces and finish his ultimate weapon, the Death Egg. Robotnik now ruled the factories, refineries and cities. He's grabbed control of everything except the seven Chaos Emeralds. The speedy hedgehog went off into the forest to set things right and the curious fox followed.
Sonic and Tails subsequently journeyed through the zones of the island to chase down Robotnik. Eventually, Robotnik made his retreat from the island. In response, Sonic and Tails took to the skies in the Tornado as they pursued Robotnik's flying ship, the Wing Fortress. Close to the Wing Fortress however, the Tornado got shot down, taking Tails with it. Having no choice, Sonic took on the fortress only to find Robotnik escaping again. However, Tails came back with a hasty rocket engine repair. With Tails and the Tornado's aid, Sonic reached Robotnik's Egg Shuttle, grabbed onto it, and managed to enter the newly-created Death Egg. Just a moment later, the Death Egg exploded in a bright light that could be seen above West Side Island. Tails later found Sonic with the Tornado in the middle of the sky and thus the island was freed.
Sonic the Fighters
In Sonic the Fighters, a section of West Side Island's Casino Night was used as a fighting arena for a fighting tournament. This arena served as the home field for Fang the Sniper. The victor of the fighting tournament eventually fought here in order to earn the right to use the Lunar Fox for a mission to destroy the Death Egg II.
After ending up in White Space, Modern Sonic and Classic Sonic separately paid a visit to Chemical Plant/Casino Night. By speeding through Chemical Plant/Casino Night, the Sonics were able to fix space, thereby restoring Chemical Plant/Casino Night from its lifeless state. When the Time Eater later got destroyed, Chemical Plant/Casino Night was seemingly return to its rightful place in the timeline.