After settling in Green Hills, Sonic is ready for more freedom and Tom and Maddie agree to leave him home while they go on holiday. But no sooner are they gone, when Dr Robotnik comes back, this time with a new partner, Knuckles, in search for an emerald that has the power to both build and destroy civilisations. Sonic teams up with his own sidekick, Tails and together they embark on a journey to find the emerald before it falls into the wrong hands.