Its basically the sequel to Sonic 3, but its not sonic 4, anyway in this game you can either play as Sonic or Knuckles in an attempt to Stop Dr. Robotnik (eggman to fans who didn't know his old name used to be Robotnik) from re-building his 'death egg', its like the death star from 'star wars' but with robotnik's face, anyway, its an excellent buy for your Sega Genesis, oh and if you buy the original sonic, sonic 2 or 3, you can lock them on to this cartridge (since sonic & Knuckles is a lock-on cartridge) you can unlcck special features.