With the Sonic the Hedgehog movie sequel, appropriately titled Sonic the Hedgehog 2, having wrapped in Toronto we now have an official plot synopsis from Paramount:
After settling in Green Hills, Sonic is ready for more freedom, and Tom and Maddie agree to leave him home while they go on vacation. 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 civilizations. 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. Being directed by Jeff Fowler, and starring Jim Carrey, James Marsden, and Ben Schwartz.