Dr. Robotnik has assumed control of Mt. Mobius and turned it into a mechanical base. Utilizing energy produced by the magma flowing under the volcano, this new monstrosity (the Veg-O-Fortress) has the power to transform helpless animals into robot slaves at an astounding rate. Sonic the Hedgehog and Tails fly onto the scene to mount an aerial assault, but Sonic is knocked off the wings of Tails' airplane by a blast from the fortress. He falls into the water, but is rescued and taken to the subterranean levels of the Veg-O-Fortress. The fortress must be destroyed from the inside-out, and the only way to make that happen is to trigger an eruption in the volcano it's built on. Sonic knows this can be done by removing the Chaos Emeralds that keep the volcano stable. Robotnik, however, is also aware of the fragile relationship that exists between the Emeralds and the mountain, and he's set up an elaborate Pinball Defense System to make sure the precious jewels don't go anywhere.