"Dr. Eggman’s (AKA Dr. Robotnik’s) Death Egg was once again blasted by Sonic, crash-landing on the peak of a volcano on the Floating Island.
Dr. Eggman is still at large, and Sonic can’t allow him to get his hands on the Master Emerald and repair the Death Egg. Sonic must also keep Knuckles off his back but Knuckles has problems too. As guardian of the Floating Island and all the Emeralds, Knuckles must do his part to keep the island safe. While they’re going the rounds with each other, who will stop Dr. Eggman?"
Sonic 3 and Knucles is the Genesis version of the game Sonic & Knuckles where it could be combined with the cartridge of Sonic the Hedgehog 3 through lock-on technology to get a different game. This is a digital release of the combined version as originally intended. The player first goes through Sonic 3 and then through Sonic & Knuckles as any of the three characters: Sonic, Knuckles or Tails. It also has small changes to the level design to fit all three characters. The combined version allows the characters to collect all super emeralds to have them transform in Hyper Sonic, Hyper Knuckles and Super Tails respectively, each with their own special ability, which is not possible in the two games separately.