a platform game in the Sonic series released for the Sega Mega Drive/Genesis, accessible by physically joining the cartridges of Sonic 3 and Sonic & Knuckles. The resulting game was how the developers originally intended the games be played.
Developed in the US in 1994, Sonic 3 & Knuckles is platform video game in the Sonic the Hedgehog series for the Sega Mega Drive/Genesis.
It is a sequel to Sonic the Hedgehog 2 and follows on directly from the end of that game.
The story begins as Dr Robotnik's Death Egg crash-lands on a floating island. Sonic & Tails are in hot pursuit, but before they get far they encounter Knuckles the Echidna: a new character who (having been tricked by Dr Robotnik into believing Sonic is the bad guy) promptly steals away the Chaos Emeralds.
Our heroes must retrieve the stolen Emeralds, dodge or work around Knuckles' traps, whilst ultimately working to stop Dr. Robotnik from relaunching his ship, the Death Egg.
After thwarting one attempted launch and landing in Mushroom Hill Zone, Sonic and Tails manage to sneak up on Knuckles as he leaves a Warp Ring and runs off. Our heroes enter the warp ring, and, upon arriving in the Hidden Palace, the Chaos Emeralds are transformed into unpowered Super Emeralds, as they surround the Master Emerald.
Meanwhile, Dr Robotnik has learned of the Master Emerald from it's protector (Knuckles), and how it keeps the entirety of Floating Island levitating. He plans to steal it in order to provide his Death Egg the levitation ability, and enough power to reach Orbit again.
Our heroes must now activate all the Super Emeralds, unleashing the full potential of the Master Emerald and granting them powers beyond that of the Chaos Emeralds alone, and try and prevent Dr Robotnik from stealing the Master Emerald.
The game developed was too large for the standard Mega Drive cartriges at the time, and was deemed to costly to develop a new cartridge. The developers made the decision to split the game into two separate games: Sonic 3, and Sonic & Knuckles. The Sonic & Knuckles cartridge featured "lock-on technology" that allowed other cartriges to be placed on top of it. As such, Sonic & Knuckles could be joined to Sonic 3, allowing the game dubbed Sonic 3 & Knuckles to be played.