When Ben isn't tall enough for a ride at a water park, he decides to change into one of his alien forms to get even with the ride attendant. Unfortunately for him, the Omnitrix malfunctions and he ends up as Grey Matter. To make the situation worse, he is captured by an alien fanatic name, who decides to trade him in for fame and glory. Now Ben, as Grey Matter, must find a way out of this predicament, after being given to a secret organization for testing.

With Vilgax's injuries fully-healed, he decides to retrieve the Omnitrix personally. Ben gets a message from Vilgax in a dream about his imminent arrival. This worries Grandpa Max greatly, and he insists that they get to Mount Rushmore as quickly as possible. Unable to pinpoint the exact location of the Omnitrix, Vilgax begins to lay waste to nearby Rapid City in order to flush its holder out. He succeeds, despite Grandpa Max's protests, and manages to capture Ben. Meanwhile, Grandpa Max makes it to Mount Rushmore with Gwen and shows her a huge facility beneath the mountain. There, he's stored a weapon that can defeat Vilgax. Unfortunately, the weapon fails, leading to a showdown between Vilgax and the combined Tennyson family.