Sonic: After the Sequel (abbreviated Sonic ATS), is a fan game created by LakeFeperd that is meant to explain the events that may have taken place between Sonic the Hedgehog 2 and Sonic 3 and Knuckles. In it, the player plays as Sonic and Tails, racing through numerous zones as Sonic and Tails go out to stop Dr. Eggman from crashing the Death Egg into (and attacking), the new, mysterious island known as Angel Island. It is the sequel to Sonic Before the Sequel.

After the events of Sonic 2, Sonic is caught by Tails, who is piloting the Tornado. The duo then sees an island with a large metal ring hovering near its snow-covered mountain. They then decide to go and investigate this new island. After getting through Horizon Heights, Sonic encounters a small robot resembling Tails. It then activates and attacks Sonic, and after a brief duel, it's revealed to be a trap by Eggman. Sonic is trapped in a magnetic forcefield, unable to stop Eggman from stealing the Chaos Emeralds. Just before Eggman could get his hands on them, Tails knocked the emeralds away, sending them back to the Special Zone in the process. Eggman then makes a hasty retreat to his temporary base, Storm Station Zone, with Sonic and Tails in hot pursuit.