At Yellowstone National Park, Grandpa Max has just finished setting up the RV. Both he and Gwen are happy to be there. Ben, on the other hand, would rather be somewhere else. Still not happy with his grandfather's idea of dinner, he is trying to turn into XLR8 so he can get a burger. Gwen opts to help once Grandpa Max eats a worm he finds under a log. Instead of producing the desired form, the Omnitrix transforms Ben into an entirely new form: Cannon Bolt, an armadillo-like alien. It's a rather clumsy form at first, and can't seem to stay upright. As Cannon Bolt tries to figure out what he can do, a meteor flies overhead and crashes a short distance away. Cannon Bolt turns in the direction of the crash and falls over again, but this time ends up covered in a hard shell and begins rolling towards the crash site.
At the crash site, the meteor begins to crack apart as the three arrive. It breaks apart to reveal a large, reddish, tick-like alien within. A ship arrives shortly after, and three humanoid aliens exit the ship riding platforms that fly through the use of an energy ball positioned directly below them. The first is a tall, pale-skinned alien with six, tentacle-like feet. The second is shorter with six arms and yellow skin. He can apparently hear and understand what the giant tick is saying, though it never makes any noise resembling speech. The final alien is of medium height, has four eyestalks, and carries a large ax. The aliens are worshippers of the giant tick, referring to it as the "Great One," and claim that it is here to purify the planet. They also identify Cannon Bolt as an Arburian Pelarota, whose planet has also been purified. They then clarify the meaning of purification. Apparently, the Great One will devour the planet from the inside out, destroying it. As the Great One begins the process of devouring the planet, Cannon Bolt tries to attack them, but the ax wielding worshipper merely bats him away and into the forest nearby. Grandpa Max and Gwen chase after him.
In the forest, Cannon Bolt bounces off of various objects before coming to a stop and reverting to human form. By this point, the Great One has poisoned much of Yellowstone, converting the landscape into a decayed wasteland with gray tentacles sticking out of the ground. The tentacles attack the trio and attempt to drag them underground. Luckily, Ben is able to transform into Stinkfly and rescue Grandpa Max and Gwen. He then flies off with them in tow.
As he flies them over the park, they pass by the Old Faithful geyser, which now resembles a giant zit that is about to burst. At the same time, the small, yellow worshipper tells his friends that the Great One would like them to witness his power, which happens to be the geyser. They meet Stinkfly along the way, this time unsure of what he is. Stinkfly drops Grandpa Max and Gwen off so he can fight the worshippers. He plugs the geyser as he intercepts them, which they aren't happy about. He does well at first, dodging their attacks easily and managing to make them lose track of him. Unfortunately, when he tries to attack them from above, he reverts to human form in mid-air. Luckily, he lands on the yellow worshipper's platform, knocking him off of it. The yellow worshipper survives the fall, landing on the softened ground below. Ben then learns to fly the platform and begins a chase with the other two worshippers, eventually causing them to crash into Old Faithful.
With the worshippers out of commission, Ben turns his attention to the Great One, which has grown substantially in the short time since it began its feeding. One after another, Ben tries to use his alien forms to destroy it. After trying every one except Gray Matter and Cannon Bolt, Ben hasn't so much as scratched it. With the Omnitrix out of power, the three are at the whim of the worshippers when they return. Tossing sponges to them, the tall worshipper orders them to clean the Great One, claiming it must be purified before the planet is. Ben would rather not, but Grandpa Max tells him and Gwen to play along for now.
As the three clean the Great One, it begins to shed, revealing a yellow shell underneath its reddish exterior. Several large, purple pores also push their way out of the shell, spitting acid at random intervals. The falling pieces of shell almost crush Ben, Gwen, and Grandpa Max. With no options left, Gwen tells Ben to use Cannon Bolt. Ben doesn't want to, since the alien seems useless and he isn't sure how to become it. However, with nothing left to do, Ben tries anyway. He succeeds, despite not knowing how.
Upon seeing Cannon Bolt again, the worshippers attack him. Cannon Bolt is able to avoid them by rolling away. As they fire on him, several of the worshippers hit the Great One. Their attacks bounce off of its shell, giving Grandpa Max the idea to use the shell fragments as shields. The shell fragments serve him and Gwen well, blocking both the lasers and the occasional acid releases of the Great One, but they are eventually trapped in a pit made by the acid. Just before they are finished off by the worshippers, Cannon Bolt jumps in to save them, causing the worshippers' attacks to bounce harmlessly off of his armor. Noticing this, Cannon Bolt uses it to his advantage, bouncing their attacks back at them and knocking them out.
With the worshippers once again out of the picture, Cannon Bolt turns on the Great One. His attacks are equally ineffective, but the pores on the Great One allow him to attack it from the inside. Once inside it, he bounces around its body, destroying its bones and organs. It then dies in an explosion of purple slime.
Seeing that which they worship destroyed, the worshippers turn to Cannon Bolt for a new thing to worship. Gwen comments on their loyalty or lack thereof. Cannon Bolt then reverts to human form. The worshippers would rather not place their faith in an inferior human, so they depart. Shortly after they leave, the landscape returns to its normal state, though why it does so is left unclear. After all the excitement, Ben finally thinks that some time outdoors will be good for him, especially since there's no mess to clean up. Unfortunately for him, Grandpa Max's RV is still covered in the remains of the Great One, and he's stuck cleaning it up.