While visiting Flicky Island, Sonic & Knuckles finds that the Flickies, his small, feathered friends that can travel between parallel worlds, were captured and turned into badniks (the robot enemies of Sonic) by Dr. Robotnik in his never-ending quest for the Chaos Emeralds. Seven levels separate Sonic & Knuckles from the final showdown with Robotnik.
Sonic & Knuckles uses an isometric view, where Sonic can move not only forwards and backwards, but also left and right. Gameplay is much slower compared to other games in the series, and the number of badniks is reduced to five in each section. The player must collect all five Chaos Emeralds by leading the animal friends to an interdimensional ring.
Unlike previous games, time isn't a requirement: if the player completes a level in more than 10 minutes, there would be no time bonus at the end. To collect all seven Chaos Emeralds, Sonic & Knuckles requires finding Tails or Knuckles and offering them at least 50 rings.
Sonic & Knuckles was released for the Sega Genesis in 1994.