When his mother brings home a guinea pig instead of the dog he has always wanted, fifth-grader Rufus is not happy--until the rodent starts acting exactly like a dog.

A constant worrier, Franklin Endicott earns a fresh cause for alarm when he accompanies Eugenia Lincoln to her favorite thrift shop and leaves with a mystery to solve.

While his human Elliot adjusts to his new home and stepfamily, Gizmo, an evil genius guinea pig, searches for a new evil lair where he can plot to take over the world, but Gizmo is constantly thwarted by Elliot's little stepsister Jasmine (who likes to play dress-up), Abuela (who may or may not like to eat cavies), and Wedgie (a Corgi superhero and Gizmo's nemesis).

When his friend Soldier hurts his head, Little Whistle tried to find something to make him feel better so he will be able to read the other toys their bedtime story.

"The guinea pigs are having a party, and like many children's birthday parties, things aren't going perfectly. But in the end everyone has a good time and the party is a success!"--

Sasspants, the reluctant guinea pig private investigator, is drawn into another mystery by Hamisher the hamster who believes there is a ghost in the pet shop.

"When Wonder Pig starts to act strange, Super Turbo gets suspicious. Is it possible that the superhero guinea pig is actually working undercover for the evil rat Whiskerface?"--

Opal's new friend Daphne seems to be the best at everything she does, but Toot and Puddle see another side of her.

Sasspants, a guinea pig, reluctantly agrees to act as a private investigator when Hamisher the hamster begs for her help in discovering who is stealing sandwiches from the pet shop's befuddled owner.

Mr. Gilbert brings in a guinea pig as a class pet, but it looks so lonely he brings another, and before long the class is getting an unexpected lesson in addition--and a very full cage.