Sonic the Hedgehog has been defeated by a powerful mysterious being known as Infinite, who has been altered and corrupted by Dr. Eggman using a mysterious gem, known as the Phantom Ruby. After his defeat, Sonic disappears, leaving Eggman free to conquer 99% of the world with a massive robotic army and his new allies, who are lead by Infinite. During his takeover, Knuckles the Echidna forms a resistance group to rebel against Eggman's rule. The resistance must find Sonic, defeat Eggman's army and liberate the world from his clutches, before Eggman's plan completes in 3 days, which will exterminate all of the members of the resistance forever.
—Gravemind