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As fun as Sonic 3 was, the fact that it contained only six levels and a fairly inconclusive ending, leaving a lot of questions unanswered, was enough for people to.. well, start asking questions. Namely "what happened to Knuckles?", "how come I don't get to play as Knuckles?", and, for those that still hadn't quite caught on yet, "what, Knuckles is a boy??" (yes, because of his dreads, some people initially thought he was a girl. I remember). Little did they know, however, that these questions would all be answered soon enough. Sonic 3 was originally designed to be quite a large game, eventually getting so large that it couldn't be squeezed onto a single Mega Drive cartridge, even with the aid of Yuji's trusty can of WD40. Compromises were made and it was decided that this particular chapter of Sonic's legacy should be the first, and (until Sonic 4 finally arrived sixteen years later) only to come in two parts, both being developed almost in unison with each other, which is why traces of S&K material, such as level names and music, can be found buried away in Sonic 3. This wacky idea allowed more levels between them, together creating the biggest, baddest Sonic game imaginable.
Sonic 3 introduced Knuckles as Sonic's latest rival, giving rabid fans a taste of their new spiny ant-eating anti-hero, but denied access to the control over him. Just as they were about to rip their hair out in frustration, in swooped Sonic & Knuckles and the day was saved. Players could choose between hedgehog and echidna (though not fox this time) to tackle the new levels on offer, and the two characters were designed to play in very different ways. Sonic may have had a little more speed and a higher jump, but Knuckles could glide and climb walls, and knock down otherwise impenetrable barriers with the mere flick of his wrist, allowing their journeys to differ via exclusive areas and routes specific to each character.
At some point along the way, someone had the biggest stroke of genius yet though, cementing Sonic & Knuckles as a very unique game, mostly for its cartridge. What about, they pondered, if we could connect Sonic 3 and Sonic & Knuckles together, so you can play the whole game from start to finish, as if it were actually one, complete adventure? Lock-on technology - not seen in any other game on the system or any other for that matter - was born!
There was an opening slot on top of the unusually-shaped cartridge that allowed you to plug another cartridge into it, from which Sonic & Knuckles could take certain data and use it with some of its own to create a fusion of the two games. Sonic 1, 2 or 3 could be plugged in there to unlock what were - kind of - three whole new games! Cross-breed S&K with Sonic 2 for example and you can now play through the whole of Sonic 2 as Knuckles! The age and wisdom of Sonic 1 made things a bit difficult, but it wasn't left out entirely, as plugging that game in allowed you access to a stupidly large amount of new automatically generated "Blue Sphere" Special Stages, as a bonus. Most importantly though was the ultimate game - Sonic 3 locked on to Sonic & Knuckles to create "Sonic 3 & Knuckles". Like merging two large pieces of sacred stone broken in two, all of Sonic 3's features and levels were kept, and S&K's levels were added on seamlessly. Not just that, but all three characters were playable in almost any of the stages (that means Knux in Sonic 3 and Tails in S&K!), and they could access new routes and get to places you had no idea even existed before, plus many exclusive scenes and other little surprises were bundled in too! At last, Sonic's third Mega Drive outing could be complete!
As the European box art advertises, the S&K cartridge has another slot on top of it, into which you can plug Sonic 2 or 3 to unlock whole new variations of these games, including the ability to play as Knuckles throughout both.
The game shipped well in time for it to be in my 1994 Christmas stocking, though unlike other Mega Drive games, its unique shape meant that it had to come in a silly little card box, like the Nintendo games of old.