Knuckles the Echidna is a game released in June 2014 for the Wii U, Xbox One and PS4, with later releases on the 3DS and PS Vita. The game follows Knuckles the Echidna, one of the heroes in the Sonic series. It is the first recent game with the main character being a secondary Sonic character since Shadow the Hedgehog.
The game (obviously) follows the story of Knuckles the Echidna, whom is strolling through the Green Hill Zone days after the events in Sonic Team F.A.S.T, therefore, the Green Hill Zone is destroyed. Dr. Eggman appears in a giant robot suit and kneels down to Knuckles, explaining how he destroyed the Green Hill Zone in an attempt to destroy Sonic, then he jokes and says the killing was successful. Knuckles doesn't believe it and tries to destroy Dr. Eggman, but the doctor picks up Knuckles and throws him a far distance away. Upon hitting the ground, Knuckles falls unconscious.
Knuckles wakes up in a futuristic-city area known as Future City Zone, although the name is not mentioned in the game, and the city is simply known as, "that city". You can then take control of Knuckles and explore, although your main target is Omochao. You explain that the hit wiped out your memory, so Omochao gladly teaches you the controls of the game, also telling you where Dr. Eggman is in the process using his R.T.S, or Robot Tracking System. The game then starts you off in the first level of Future City Zone, and you are free to advance through the world as you please.
Future City Zone: 10 Levels, one boss tutorial.
Robot Hill Zone (destroyed Green Hill Zone): 5 Levels.
Sky Scraper Zone: 12 Levels, one boss level.
Eggman Woods Zone: 2 Levels, one boss level.
Tiny Land Zone: 4 Levels, one boss level.
Shadow Land: 13 Levels, one boss level.
Eggman Industry Zone: 7 Levels, one final boss.
Bonus Level: Chao Gardens Return!
There is no 4P mode in Knuckles the Echidna (that's for an upcoming fan-made Sonic game!). Instead, there's a 2P mode where you can replay levels you've played in the story with a friend, as well as raise Chao together.