Coop 's adventures continue as he finds new ways to prove that his sister's tiny hairless cat is an evil alien bent on destroying him. While Kat is creating and implementing new diabolical plans, Coop can be found protecting his new found crush, campaigning to humiliate Kat, and attempting to prevent Kat's evil plans, like the one to turn Bootsvillie's entire water supply into milk.
When Millie tickles Mr. Kat's tickle spot he goes into a blissful state that quickly anger's Kat's boss. So, Kat's boss sends out a robot to destroy Millie; Coop, Kat, and Dennis must work together, and stop the robot from destroying Millie and themselves.
Kat's swarm of flea-sized mind control robots hop into the ears of the people of Bootsville and soon they all want to destroy Coop. What will Coop do to save himself from being destroyed by the people, get the flea-sized mind controls out of their ears, and save everyone?
Millie brings home the class hamster, Mr. Cheeks. Turns out that Mr. Cheeks is an evil alien, just like Kat, and Kat and Mr. Cheeks are enemies. Can Kat save the whole world from being invaded by Mr. Cheeks?
Coop decides to let Kat have free rein to cause whatever destruction he wants. When Dennis sees Kat doing something big, will Coop save the whole world from Kat, or let Kat get away with invading the whole world?
Coop becomes Dennis' wing-man for a change, as they go in search of the 'sewer-croc-baby'. Meanwhile, Kat goes into the drain also to get his experiment that blasts through things, and runs into the 'sewer-croc-baby'.
Kat's new invention makes Old Lady Munson's gnomes come to life, and they choose Coop as a leader because Coop have a cone stuck on his head, that makes him look like a gnome. What will their new king do?
Mr. Cheeks come back to revenge Mr. Kat, but Lorne and Harley got him to help them in the homework. Meanwhile, Millie wants to get a "First aid" medal, with help from Kat; Coop and Dennis go to rocket science camp.
Coop's 11th birthday is here, and everyone is coming to his party, and Phoebe is trying to get rid of Fiona so she can have Coop to herself. Meanwhile, Kat is trying to enjoy a virtual picnic in a secluded spot with his girlfriend, but the kids keep crashing Coop's birthday.
Coop goes on a train to buy something from a man named Old Herman, Unfortunately it turns out to be the evil alien hamster, Mr. Cheeks. When the train leaves the desert without Coop and Kat on it, they must find a way to get back on the train to get back home. Can Coop and Kat stop Mr. Cheeks from making the train crash, and eliminating everyone?
Kat coughs up a trio of living hairballs, which he orders them to attack Coop and Dennis. When the hairballs get out of hand, Coop, Kat, and Dennis must stop the hairballs from destroying the whole world.
Coop takes on the teen aged bully who pedals the Curly Swirl Ice Cream cart, and forgets to pay attention to Kat's latest evil plans. Will Coop forget all about the teen aged bully, and stop Kat with his evil plans?