Sonic Chaos is a platform game developed by Aspect and published by Sega. The game was released for the Master System in October 1993, and for the Game Gear the following month. The Game Gear version was re-released in 2003 as a hidden game in Sonic Adventure DX for the GameCube and Windows and as an included game in Sonic Mega Collection Plus on the PlayStation 2, Xbox and PC. In 2009, the Master System version was released on the Virtual Console.
Sonic Chaos was the first 8-bit Sonic title to feature Tails as a playable character, and also the first in the series to allow the player to manually control his flight. A sequel, titled Sonic Triple Trouble, was released in 1994.
The plot begins with the theft of the chaos emeralds from South Island by Dr. Robotnik. Since the peace and integrity of the island depend upon the presence and unity of all six emeralds, the theft causes earthquakes and turmoil. Sonic and Tails learn from Flicky the Bluebird that the emeralds have been taken from the island's North Cave. They then chance to see the evil Dr. Robotnik flying away with the red chaos emerald. In order to prevent South Island from eventually collapsing into the ocean, Sonic and Tails set out to reclaim the emeralds.
Overall, the gameplay is similar to previous 8-bit Sonic games, though the game allowed players to engage for the first time as either Sonic or Tails. For the first time in the 8-bit series, the player may perform the Spin Dash introduced in the 16-bit version of Sonic 2. By holding up and pressing buttons, Sonic can perform the Super Peel-out introduced in Sonic CD, while Tails is able to fly, allowing players to control his flight for the first time. While playing as Sonic, players can collect rocket boots which allow him to fly through the air for a short time. He is also able to enter Special Stages. Tails is slower than Sonic and can't enter Special Stages or use rocket boots, but he starts the game with more lives and continues.
Unlike previous 8-bit Sonic games, the Chaos Emeralds are mostly located in Special Stages available while playing as Sonic. In order to enter a Special Stage, Sonic needs to collect 100 Rings in one act (the player also will be awarded an extra life for collecting 100 Rings). Successfully completing the Special Stage allows Sonic to collect a Chaos Emerald. Failing to complete the Special Stage will put Sonic to the next act, without the Emerald. Robotnik himself holds the sixth Chaos Emerald, and the player needs to beat him in order to win it back. The objective is the same for both characters, with the same ending; effectively making Tails an "Easy" mode. The are six principal Zones to complete: Turquoise Hill; Gigalopolis(Gigapolis in Game Gear); Sleeping Egg; Mecha Green Hill; Aqua Planet; Electric Egg.