Sonic Chaos is a fan game and re-imagining of the 1993 Sega Game Gear and Master System platform game of the same name. Based on the current style of 2D classic Sonic games inspired by Sonic Mania, Sonic Chaos updates the 8 bit visuals of the original and adapts a play style more closely resembling the classic 16-bit games.

The storyline takes place on South Island, the resting place of the Chaos Emeralds. While Sonic and Miles "Tails" Prower were gone on an adventure, Dr. Eggman managed to procure one of the Chaos Emeralds. As a result, the rest of the Emeralds scattered around the island. Without the Emeralds' power, South Island began sinking into the sea. Hearing rumors of Eggman's plot, Sonic and Tails returned to South Island, only to find it in chaos. With no time to lose, the duo rushed off to stop Eggman's plan and save the island by regaining the Emeralds.

In the western releases, the story had its premise simplified. Following his twisted ideas for conquering the world, Dr. Robotnik stole the red Chaos Emerald to create nuclear bombs and laser weapons. As a result, the rest of the Emeralds lost their balance and flew into a parallel universe. Without the Emeralds' power, South Island began sinking into the ocean. Sonic and Tails thus set off to stop Dr. Robotnik and restore balance to the universe by getting the Emeralds back.