This game is what you get if you take Sonic the Hedgehog 2 and lock it onto Sonic & Knuckles. As the name suggests, what you get is simply Sonic the Hedgehog 2 with Knuckles the Echidna as the playable character. The levels remain mostly unchanged, save for the addition of a few monitors that can only be reached as Knuckles, whose abilities from Sonic and Knuckles (gliding, climbing walls, and jumping a little lower than Sonic) remain intact.

In this run, you'll see the zones of Sonic the Hedgehog 2 (colloquially referred to as West Side Island in the Japanese manual) like you've never seen them before, as Knuckles uses every trick in the book to collect over five thousand rings, with only 11 throughout the game eluding his grasp. (For the time being, anyway...)

We also have runs with Sonic and Tails and Knuckles in this game.