This, is a commercial promoting Sonic & Knuckles for the Sega Mega Drive/Genesis, from 1994. Sonic & Knuckles is the second half of Sonic 3 & Knuckles, and when Sonic The Hedgehog 3 is locked onto the cartridge, it unlocks the full game (Sonic The Hedgehog 3 & Knuckles). Putting Sonic The Hedgehog 2 onto the cartridge lets you play as Knuckles The Echidna in Sonic 2. Putting Sonic The Hedgehog on the cartridge, on the other hand, unlocks the full version of the Blue Sphere mini-game (special stages in Sonic 3 & Knuckles).

https://info.sonicretro.org/Sonic_%26...