When the Mahoney family hires Henry the Critter Sitter to watch their dog, cat, bird, fish, frog, and snake, he thinks he is up for the challenge since creature control is his game, but the pets have a different idea.

Ellie Ultra has a lot of responsibilities as a superhero. So when the chance to become Hamster Helper for the class's hamster, Squeak, comes around, Ellie isn't so sure she wants to add another thing to her list. But her name is picked and now it's up to Ellie to make sure Squeak is taken care of. All is fine until a mishap in the lab happens and suddenly Ellie's not so big any more-literally! A tousle with the Ultra Shrinker makes Ellie three-inches-tall....

On the very first day of summer, Hank's dad runs over his bike, because Hank was irresponsible and left it in the driveway at night. Needing money for a new one, Hank starts a pet sitting service. His first pet is Otis, the very big, trouble-making dog.

Hank and his pet-sitting business must deal with a very scared dog named Otis, who gets especially scared during thunderstorms.

Continuing his efforts to make enough money to buy a new bike, Hank the pet sitter takes a job watching Pickles the pig, with help from his friend Janie.

Twelve-year-old Katie now knows her neighbor Madeline is also the Mousetress, the city's most feared villain (or most misunderstood superhero), and she is ready to become the super sidekick she always wanted to be.

Katie the Catsitter is now an official superhero and it is up to her and her friends to save the city from corruption.