Developed in the US in 1994, Sonic & Knuckles is platform video game in the Sonic the Hedgehog series for the Sega Genesis.
It featured 'lock-on' technology that enabled it to be attached to other Sega Genesis cartriges, most notably Sonic's previous games, Sonic 3, and Sonic 2.
Story - Sonic
It is a sequel to Sonic the Hedgehog 3 and follows on directly from the end of that game.
Sonic has just thwarted one attempt by Dr Robotnik to relaunch his Death Egg, but lost the Chaos Emeralds in the process. After The Death Egg crash-lands in the Floating Island's dormant volcano, Sonic attempts to make his way back to it, in order to finish the job once and for all.
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.
Sonic must retrieve the lost Emeralds, dodge or work around Knuckles' traps, whilst ultimately working to stop Dr. Robotnik from relaunching his ship, the Death Egg.
Story - Knuckles
Knuckles' side of the story follows on after Sonic's ends.
Knuckle's is taking a well deserved afternoon break, when he is discovered and attacked by a surviving robot, called "Egg Robo".
Knuckles must now chase down Egg Robo, whilst finding and keeping the Chaos Emeralds safe.
The game was originally supposed to be a continuation of Sonic 3, however time constraints and cartridge costs forced the game to be split into two parts. As such, Sonic & Knuckles is closely tied to its predecessor Sonic 3, and the two games can be physically joined to continue the saga as originally planned.
Sonic & Knuckles can also be joined to Sonic 2, allowing players to play through that game as Knuckles.