Chilli and Bingo are at the hospital because Bingo isn’t feeling well. The doctor asks Bingo to take a deep breath, and Bingo lets out a big one. Chilli and Bingo ask if they can go home soon, but the doctor says Bingo needs to stay for another night. Bingo wants to go home, but then they get a notification on the tablet – it’s a video called Bumpy and the Wise Old Wolfhound.
The video starts with a girl named Barnacus, who lives in Purple Underpants Town and is wearing purple underpants. She’s sad because she doesn’t have a puppy, so she visits the corner shop and tells the shopkeeper she wants one. The shopkeeper suggests she should wish upon a star. That night, Barnacus wishes upon the Grand Star for her very own puppy. The next morning, her wish comes true – she gets a cute blue puppy that can bite people, and she names it Bumpy. Barnacus takes Bumpy to the corner shop to buy carrots, since Bumpy loves carrots and biting people. As they play in Purple Underpants Park, Bumpy starts to feel unwell.
As the days go by, Bumpy gets worse and develops rainbow dots. Barnacus tries to get help from the fisherman at the Purple Underpants Harbour, the builder at the construction site, and even the shopkeeper, but no one can help her. She then visits the bakery, where the baker tells her about the Wise Old Wolfhound. Barnacus and Bumpy go to the Purple Underpants Fortune Tent to meet the Wise Old Wolfhound, who says Bumpy is sick but that she’s not a vet. However, she has some special medicine – a pair of purple underpants from someone who has never been sick. Barnacus searches for the underpants but finds out that everyone in town has been sick at some point. Eventually, Barnacus realizes that being sick is a part of life. Later, Bumpy recovers, and in a funny moment, Bumpy bites the shopkeeper’s bum, which has bumworms.