Sonic & Knuckles is a platform game developed by Sonic Team and published by Sega for the Sega Genesis. Released on October 18, 1994, it is the fifth installment in the main Sonic the Hedgehog series and a direct sequel to Sonic the Hedgehog 3, which released earlier that year. It follows Sonic the Hedgehog and Knuckles the Echidna in their respective quests to save Angel Island; Sonic tries to prevent Dr. Robotnik from relaunching his orbital weapon, the Death Egg, while Knuckles scuffles with Robotnik's minion, known as EggRobo.
Sonic 3 and Sonic & Knuckles were originally intended to be released as a single game. However, due to time constraints and the cost of a large-memory-capacity cartridge, Sega split the project in two. The Sonic & Knuckles cartridge uses "lock-on technology" that allows the game to connect to the Sonic the Hedgehog 2 or Sonic the Hedgehog 3 cartridges, combining elements from either game.
Sonic & Knuckles was met with positive reviews. Critics were impressed with the lock-on technology, although criticizing its similarity to its predecessor. It has since been rereleased in various compilations and on digital platforms such as Xbox Live Arcade, Virtual Console, and Steam. Direct sequels, Sonic the Hedgehog 4: Episode I and Episode 2 were released decades later, starting in 2010.
The game follows Sonic and Knuckles in their respective quests to save Angel Island. Sonic's side of the story picks up immediately after the events of Sonic 3, where Dr. Robotnik's orbital weapon, the Death Egg, is damaged in a battle with Sonic and crash-lands back onto Angel Island
Sonic travels through each zone looking 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.