When Sega debuted Sonic the Hedgehog 3 in February 1994, it was a success – but it was only the first part of that game’s story. The second half of the tale arrived that October, with the debut of Sonic & Knuckles.

Sonic & Knuckles had been designed by Sega originally to be a part of Sonic 3, but limitations in technology at the time caused the development team to split the game in two. This resulted in the development of the “Lock-On Technology” cartridge, in which a second cartridge could be inserted into the Sonic & Knuckles cart. The technology would combine elements from the two carts together to create new level layouts and access the Special Stages in Sonic & Knuckles to collect the Super Emeralds. Inserting the Sonic 3 cartridge essentially allowed for players to enjoy the game as the developers had originally intended, with Knuckles becoming a playable character in the base Sonic 3 game. Players could also insert Sonic 2, which let them play through that game using Knuckles’ abilities.

The game’s story picks up right where Sonic 3 leaves off, with the villainous Dr. Robotnik’s weapon known as the Death Egg crashing down onto Angel Island. After a brief confrontation between Sonic and Knuckles – in which Knuckles believes that Sonic is trying to steal the Chaos Emeralds – the two decide to team up in order to destroy the Death Egg and take down Robotnik together.

Despite basically just being the second half of Sonic 3, Sonic & Knuckles saw wide praise upon its release, with the Lock-On Technology on the cartridge seeing a lot of positive attention specifically. The main gripe that critics had about the game was that Sonic & Knuckles didn’t do much to stand on its own, separate from its predecessor. Sonic & Knuckles eventually sold more than a million copies in the U.S. alone, and the game’s legacy has carried on into future Sonic titles, with many stages seeing remixes in more current titles like Sonic Generations and Sonic Mania.