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 and has access to unique shield abilities, whereas Knuckles can glide and climb most walls. The levels also include cutscenes that differ based on the character selected, as Sonic and Knuckles are opponents 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.
Plot
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, which takes place after the events of Sonic's campaign, 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.
Reception
Critics praised Sonic & Knuckles, despite its similarity to its predecessor. The four reviewers of Electronic Gaming Monthly named it their "Game of the Month". They lauded the lock-on technology and remarked that despite that being "more of the same, it still is an exceptional game." A reviewer of GamePro, who gave it a perfect score, commented that the ability to play as Knuckles makes it essentially two games on a single cartridge, the game is more challenging than Sonic 3, and the ability to hook the cartridge up to Sonic 2 and 3 makes those games "worth playing again." Next Generation called it "the same Sonic game that Sega has sold for the last three years, just wrapped up better and with a prettier ribbon."
Critics praised the lock-on technology the game offered. Lucas Thomas of IGN said it was "a great game on its own", but the lock-on feature completely revamped the overall experience. Dan Whitehead of Eurogamer preferred Sonic & Knuckles to Sonic 3, stating that he could not fully appreciate its predecessor without its "companion piece". Sega Power's review praised the game's harder difficulty in comparison to its predecessor and the new expansion of levels, admitting that the expansion would not have been possible had Sonic 3 been a single game. Sega Magazine's review similarly praised the lock-on technology and the new innovation the unique cartridge offered, adding that Sonic & Knuckles' hidden stages and bosses would strongly add to the replay value of the combined game.
Reviewing the Virtual Console release, Nintendo Life writer James Newton praised its support for the old lock-on feature of the original release, claiming that the game does not truly shine without having purchased Sonic the Hedgehog 2 and 3 to activate this feature. Thomas praised the game for "impressive visuals that pushed the Genesis to its limits" and for the value added in the content unlocked with the lock-on technology.
The Genesis version sold 1.24 million copies in the United States.
