The Beast Inside is the twenty-ninth episode of Ben 10.
The episode starts off with the gang dropping by Max’s old college again. Max tells his grandkids that his old friend, Lester, has invited them to take a tour at the new zoo at the college. Max then goes off to get tickets from Lester, leaving the two by the Rustbucket.
Ben and Gwen take a walk around the college while looking at some tents for the animal exhibit, when suddenly a group of screaming people rush out from one of the tents. The two then see a huge, leopard-like humanoid coming out of the tent. Ben unintentionally turns into Upgrade while the mutant is causing havoc at the zoo. Upgrade merges with a patrol cart and turn it into bulldozer to try to push the mutant into a cage, unfortunately, the mutant threw Upgrade into a garbage can, causing him to turn back. Gwen then confronts the creature who was planning to go after Ben before it runs away.
Ben and Gwen follow some claw marks into a laundry room where they found Lester, who explains that he tried to stop a strange man from taking samples of the zoo’s animals before he mutated him. He also explain that Gwen reminded him of his sister when she was younger, snapping him out of the animal mutation. Ben and Gwen then investigate a mobile lab that had a blueprint for a DNA converse module. Gwen found a photo of a young Animo and his father. They then figured that Animo might still be taking samples, so they head ran towards the rare animal exhibit, but are then stopped by a guard. Fortunately, Ben was able to trick the guard, allowing then to proceed. Ben and Gwen were searching the tent until they encounter a tied up Grandpa Max and Dr. Animo. The doctor explains that after seeing Ben use the Omnitrix, it inspired him to create a device that merges human DNA with an animal’s, the Animerger.
Ben turns into Heatblast and fires at Animo, but he dodges. Animo then tried to shoot Ben using his gun, but both Ben and Gwen rolled out of the way. Heatblast bumps Animo into a cage of gorillas, to which he tried to fire at Ben again, missing. Ben manages to smack the Animerger out of the evil doctor’s hands, and then attempts to melt it. However, the heat actually causes the gun to explode. Animo tried to save his creation, but was mutated into a wolf-gorilla-octopus hybrid from the blast. Ben attempts to fight him, but he seem to get overpowered.
Animo then decides to go after the tied up Max, but Gwen was able to lead him into the State U football field, before Heatblast took over. After a bit of fighting, Gwen thinks that they can turn Animo back to normal by showing him something he cares about, which she and Ben believes is the picture from the mobile lab.
Gwen then takes off to find the photo while Ben battles the mutated Animo. Gwen then returns and throws the photo to Ben, who just turned back to normal, before presenting it to Animo. The beast was fortunately able to turn back to normal before the guards arrive to take him away. Gwen and Ben went back to the tent to free Grandpa Max as they jinx each other once again.