OK then. I'm not going into great depth with this one, but basically Sonic and Knuckles, when linked with Sonic 1, gets you this monstrous mini-game thing. You can play as either Sonic or Knuckles, through literally millions of Sonic 3 Special Stages. Sounds fun eh?
You can either collect all of the blue spheres (that is the plan) and not bother with the rings, or you can try for a 'Perfect' score by collecting all the rings as well. To gain rings from spheres you must find a group of spheres (say, in a square formation). Run around all of the spheres on the OUTSIDE of the group to produce rings. There is a point to collecting all the rings in a stage though - getting a 'Perfect' advances you ten levels instead of just one.
Completing a stage results in a cool little 'Congratulations' screen. If you get a 'Perfect' however, you'll get access to another screen that ranks how difficult that stage was. Good eh? Er, not really. It's always nice to know how skilled you are though, just to inflate your ego that little bit further.
Well, this'll certainly last in terms of longevity, but as far as innovation goes, this is near the bottom of the pile. It's just a bunch of Special Stages for crying out loud! Still, there's a password system available, and since this is merely a filler anyway (because S&K's supposed to link with Sonic 3) this 'game' is only really good to kill some time.