Sonic & Knuckles is a game/expansion released on the Sega Genesis starring Sonic the Hedgehog and Knuckles the Echidna. It is the first game to feature Knuckles as a playable character. Its success led to Knuckles becoming a playable character in Sonic the Hedgehog 3. It has since been ported to the GameCube, DS, and Virtual Console. If the player downloads both Sonic & Knuckles and Sonic 2 or Sonic 3 onto the Wii, Knuckles will become a playable character in Sonic 2 and Sonic 3.
Since Sonic & Knuckles and Sonic the Hedgehog 3 were initially developed as one game, their gameplay is similar: both are 2D side-scrolling platformers with similar level design, graphics, and game mechanics. However, in Sonic & Knuckles, unlike in Sonic 3, the player chooses either Sonic or Knuckles at the title screen, Miles "Tails" Prower is not available to select, and the player cannot control two characters together. There is also no multiplayer mode or save feature.
The player character moves through six levels, each divided into two acts. The first act of each level ends with a miniboss fight with one of Dr. Robotnik's robots, while the second ends with a regular boss fight with Robotnik (or EggRobo in Knuckles' campaign). Sonic and Knuckles traverse levels differently: Sonic can jump slightly higher, is faster and can use the insta-shield ability which makes him invincible for a split-second, whereas Knuckles can glide, break obstacles and climb most walls. The levels also include cutscenes that differ based on the character selected, as Sonic and Knuckles are rivals for most of the game.
The game contains two types of bonus stages accessed by passing a checkpoint with at least 20 rings. The first type has Sonic or Knuckles orbit floating, glowing spheres, jetting off each one when a button is pressed, while a fence of light approaches from the bottom and will remove the player from the stage if touched. Collecting 50 rings in this stage earns the player a continue. The second type involves bouncing around a room with a slot machine in its center with the intention of winning extra lives and power-ups.
Special Stages are entered by finding giant rings hidden in secret passageways: the player is placed in a 3D environment and must turn all of a number of blue spheres red by running through them, but must avoid all red spheres, including formerly blue ones. Yellow spheres bounce the player long distances, and white spheres with red stars on them make the player walk backwards in the opposite direction. Completing a Special Stage earns the player a Chaos Emerald; collecting all seven Emeralds allows the player to turn into Super Sonic or Super Knuckles, more powerful versions of the characters.
Originally, Sonic & Knuckles wasn't a separate game from Sonic 3. The two games were supposed to be on one cartridge, but due to technical limitations, they had to release as separate cartridges.
The story begins 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. In Hidden Palace Zone, Sonic fights and defeats Knuckles. The two hear a disturbance outside of the chamber, and go out to find Dr. Robotnik stealing the Master Emerald, the secret to the island's levitation powers. Knuckles attempts to attack Robotnik, but is electrically shocked in the process, and is trapped with Sonic in an underground passage. Knuckles, realizing Sonic is on his side, shows him a portal that leads them to Sky Sanctuary, where the Death Egg is relaunching. Sonic proceeds to infiltrate the Death Egg, and defeats Robotnik as Super Sonic.
Knuckles' story begins in Mushroom Hill, where he is relaxing with his animal friends but is interrupted by a bomb dropped by EggRobo. This prompts Knuckles to chase after him, leading him through most of the same zones Sonic went through. The chase ends at the damaged remains of Sky Sanctuary, where a robot called Mecha Sonic attacks Knuckles but accidentally destroys EggRobo instead. After a short fight, Mecha Sonic uses the power of the Master Emerald to achieve a Super form similar to Sonic's. Knuckles manages to defeat Super Mecha Sonic, who explodes. Sonic flies in piloting the biplane Tornado and Knuckles hitches a ride to return the Master Emerald to Angel Island. If all the Chaos Emeralds are collected, Angel Island rises upwards, into the sky. However, if the player has not collected all the Chaos Emeralds, Knuckles cannot redeem the Master Emerald, and Angel Island plummets into the ocean.
- Sonic & Knuckles was supposed to be compatible with Sonic 1, allowing Knuckles to be played. Instead, if the player were to plug in Sonic 1, they'd have access to a randomized set of Special Stages.