Origins: Sonic 3 & Knuckles - Part 3
After the events of Sonic 3, Dr. Robotnik's orbital weapon, the Death Egg, crash-lands on Angel Island. Sonic travels to Angel Island to retrieve the Chaos Emeralds to defeat Robotnik, and once again comes into conflict with Knuckles, who believes Sonic is trying to steal the Emeralds for himself. In Hidden Palace Zone, Sonic fights and defeats Knuckles, only to discover Robotnik stealing Master Emerald, the secret to the island's levitation powers. Knuckles attack Robotnik, but Robotnik shocks him with electricity. Knuckles shows Sonic a portal that leads them to Sky Sanctuary, where the Death Egg is relaunching. Sonic infiltrates the Death Egg and defeats Robotnik as Super Sonic, retrieving the Master Emerald and returning it to Angel Island as it rises back into the sky.
In Knuckles' story, taking place after Sonic's, Knuckles is attacked by EggRobo, one of Robotnik's robots. He chases him to the damaged remains of Sky Sanctuary, where Mecha Sonic attacks Knuckles, but accidentally destroys EggRobo instead. Mecha Sonic uses the power of the Master Emerald to achieve a Super form similar to Sonic's and fights Knuckles until the latter eventually defeats the mecha. Sonic flies in piloting the biplane Tornado, and takes Knuckles and the Master Emerald back to Angel Island. If all the Chaos Emeralds are collected, Angel Island rises upwards, into the sky. However, if the player has not collected all the Chaos Emeralds, Angel Island plummets into the ocean.