This cool new integration between the two games, along with the addition of a popular new character allowed both Sonic 3 and Sonic & Knuckles to sell four million copies, making them some of the best-selling Sonic games to date. The game has different endings depending on if the player has collected all of the Chaos Emeralds. Knuckles, on the other hand, chases after EggRobo in the Sky Sanctuary.
After that, Sonic runs through his own adventure pursuing Dr. The game starts with Knuckles still fighting against Sonic, only to learn after being defeated that Dr. The Sonic & Knuckles cartridge locks onto the Sonic 3 cartridge to unlock more stages and abilities. By opening the top of the Sonic & Knuckles cartridge and inserting Sonic 3 players could run through both games as one and experience additional features that weren't available in either game separately - for instance, the ability to save progress, access the previously locked Special Stages to collect 'Super Emeralds,' and unlock Hyper Sonic, Hyper Knuckles, and Super Tails. The Sonic 3 and Sonic & Knuckles cartridges combined together with Sega's patented 'lock-on technology'. Sonic & Knuckles released the same year and was actually a continuation of the plot in Sonic 3.
