This is a version of Sonic the Hedgehog 2 with Sonic & Knuckles locked on it. The title character is replaced by his rival, Knuckles. The levels remain unchanged.
There is more than just an appearance change to this game. Knuckles' abilities remain intact. If you haven't seen Sonic the Hedgehog 2 completed before, you may wish to do so before seeing what a character with a spindash and gliding can accomplish.
The run is an improvement of 5.9 seconds in-game time over the predecessor movie. The author's comments contain a whole bunch of details.
Each second link or set of encodes uses a camhack, which forces the camera to focus on Knuckles while keeping the sprites on the foreground layer.