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Sonic & Knuckles was released as an additional video game when Sonic the Hedgehog 3 had to be split into two games to meet the deadline. The Lock-On port on the top of the cartridge could be used to insert a Sonic the Hedgehog 3 cartridge (Sonic 3 for Europe), and combine the two games (using Sonic 3's SRAM) into Sonic 3 & Knuckles, while inserting the Sonic the Hedgehog 2 cartridge allows you to play Knuckles the Echidna in Sonic the Hedgehog 2; inserting any other cartridge allows access to the Blue Sphere Easter egg game.
Sonic & Knuckles carries on from where Sonic the Hedgehog 3 left off. At the end of Sonic the Hedgehog 3, after Dr. Robotnik's defeat at Launch Base, the rebuilt Death Egg fell back onto Angel Island and crashed on top of Lava Reef's volcano (retconning the ending of Sonic 3, in which the Death Egg is destroyed). Now the evil Dr. Robotnik attempts once again to repair the Death Egg, this time by trying to use the Master Emerald to fuel its engines. This all-powerful jewel, upon which the Chaos Emeralds base their powers, is guarded by Knuckles the Echidna. In Sonic the Hedgehog 3, Dr. Robotnik was able to fool the echidna into working for him, and convince him to impede Sonic in his travels; in Sonic & Knuckles, Dr. Robotnik betrays Knuckles in order to obtain the Master Emerald. Knuckles then realizes that Dr. Robotnik has been playing him all along. Both Sonic and Knuckles then agree to reluctantly work together in order to stop Dr. Robotnik, and save Angel Island. Sonic & Knuckles thus resolves the feud between the two characters, and ties up the plot of Sonic the Hedgehog 3.
Gameplay is identical to that of Sonic the Hedgehog 3, with the same power-ups available. Tails is not available as a playable character, and Knuckles (in addition to Sonic), is now playable. Both characters play nearly identically, but Knuckles has a few key differences; he is able to glide, with results in him being able to traverse levels slightly differently compared to Sonic, and he is also able to just walk through certain block barriers (that Sonic can only get past by using a Super Spin Dash). Knuckles also has his own super form available, Super Knuckles, which is unlocked by collecting all seven Chaos Emeralds. While each zone is divided into two acts (apart from Hidden Palace Zone and Sky Sanctuary Zone, that get one act each), the final zone each character is different; Knuckles ends his journey in the Sky Sanctuary Zone, while Sonic is able to progress to both the Death Egg Zone and The Doomsday Zone; the latter can only be accessed by Sonic if the player has obtained all seven Chaos Emeralds.
- Mushroom Hill Zone
- Flying Battery Zone
- Sandopolis Zone
- Lava Reef Zone
- Hidden Palace Zone
- Sky Sanctuary Zone
- Death Egg Zone
- The Doomsday Zone