Sonic and Knuckles

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 fourth installment in the main Sonic the Hedgehog series and a direct sequel to Sonic the Hedgehog 3. 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, EggRobo. Sonic the Hedgehog 3 and Sonic & Knuckles were originally intended to be a single game.
However, due to time constraints and the cost of a large-memory-capacity cartridge, Sega split the game 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.

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, landing in a volcanic crater. 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.

At Sky Sanctuary, Sonic battles against Robotnik's robotic EggRobo minions and Mecha Sonic. Mecha Sonic is left behind in a damaged state as Sonic boards the relaunching Death Egg. Robotnik uses the power of the Master Emerald to fuel a giant mech, but Sonic uses the power of the Chaos Emeralds to transform into Super Sonic. Super Sonic stops Robotnik, destroys the Death Egg, and returns the Master Emerald to its rightful place. If the player collects all the Chaos Emeralds, an ending is shown where a lone EggRobo rises from the wreckage of Robotnik's mech. This leads into Knuckles' story.

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 Sky Sanctuary, where 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 blows up. 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.

Since Sonic & Knuckles and Sonic the Hedgehog 3 were developed initially as a single title, 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 eight 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; there is a final boss fight with Robotnik at the game's end. The two characters 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, but 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. The amount of rings held by the player determines which bonus stage is accessed. This is made visible by the color of the stars floating around the checkpoint, red denoting the glowing spheres stage and yellow denoting the slot machine stage.

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. 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.