"Sonic & Knuckles" is a platform video game released in 1994 for the Sega Genesis. It is a direct sequel to Sonic the Hedgehog 3 and features the titular characters, Sonic and Knuckles the Echidna. Players can control either character, each with unique abilities, as they navigate through various levels to stop the villain Dr. Robotnik from collecting the Chaos Emeralds. The game introduced a unique locking mechanism that allowed players to connect it with Sonic the Hedgehog 3 to access additional content. This feature enabled players to experience a combined game with new levels and characters, enhancing the overall gameplay experience.