The following version of the book was used to create this study guide: Pilkey, Dav. Dog Man: Twenty Thousand Fleas Under the Sea. Scholastic Press, New York, NY, 2022. Kindle AZW file.

When Dog Man goes with the Chief on the Chief's date with Nurse Lady, he winds up chasing a squirrel while running from the cook at the Shark Shack. The cook then winds up with a lot of dogs of his own. Meanwhile, Petey the Cat is trying to find someone who will take on the job of rebuilding his lab. The first construction lady agrees to do the job. Then, she refuses when she realizes Petey is the cat who stole a lot of toilets, prompting everyone to wet their pants. Petey, who was once a villain but is now trying to be good, is disappointed that he continues to face the fallout of his past. Despite Petey's disappointment, he pushes Li'l Petey and Molly to do their best. When they are stuck on how to end a comic, Petey says they are supposed to keep trying. Sarah Hatoff gives the group a very similar message. Li'l Petey and Molly agree that it sounds better coming from Sarah.

While all this is going on, a villain named Piggy is trying to escape from the Mini Jail where he and some of his gang members have been hiding. When the jail breaks open, they make their escape. The problem is that Piggy is intent on continuing his villainous ways as a member of the group called Fleas while the rest of his group has formed a new organization – the Friendly Friends. Piggy becomes annoyed with their constant singing and cheerfulness. They join an inventor named Uncle Larry who has built an incredible headquarters for the Friendly Friends. He also has a flying submarine. Piggy is thrilled to try out all the gadgets, still with an eye toward villainy.

Reporter Sarah Hatoff has learned about the return of the Fleas. She calls in her information to the media, but a series of misunderstandings prompts a news story about “Twenty Thousand Fleas Under the Sea.” This prompts protests, and Piggy sets out to cause chaos as a way to distract everyone from the real problem – Piggy's villainy. Piggy manages to shrink the submarine and everyone on board. They wind up in a sweat gland on Piggy's nose where they have to work together to fight a mite that is much bigger than they. When they grow the submarine to a giant size, they inadvertently stop Piggy's villainous reign.

As the novel comes to a close, everyone joins in to help Petey rebuild his lab. He discovers that he has friends willing to help despite his past mistakes. Petey has tried to make Li'l Petey and Molly understand the necessity of living life to the fullest, but they do not seem to get the message until Sarah says it. After all, it always sounds better the way Sarah says it.