Doctor Undergear's Flu is the 22nd episode of Season 19.
Romeo's grandfather, Doctor Undergear, becomes sick from a flu and he needs his grandson's help to finding the ingredients for the medicine that will cure him. Then the PJ Masks and Captain Jake and his crew unwillingly help Romeo, but getting the last ingredient from Crab Island won't be easy.
The episode begins at Gear Island where inside, Doctor Undergear was teaching his grandson, Romeo, more mechanical building lessons. Just then, Undergear let out a cough as he covered his mouth with a gloved hand and continued screwing the bolt a little tighter.
But before he could finish, Doctor Undergear felt a headache coming and as he dropped his wrench, he fell backwards just as Romeo caught him and asked his grandfather if he was okay. But Undergear wasn't okay! He had a flu!
As Romeo helped Doctor Undergear get in his bed, Doctor Undergear let out a hoarse tone to tell his grandson that he needs medicine. Romeo scavenges in the cabinets, but all he could find were two empty bottles... and then a long list of the ingredients for the medicine. So after reading the list, Romeo puts the bottles and the list into his backpack and went off to find the ingredients.
Later that night at the PJ Masks' HQ, Captain Jake, Izzy, Cubby, and Skully were having a sleepover with the PJ Masks. They played video games, watched movies, and ate snacks. All was fun and games when suddenly, there was a knock at the door. Catboy goes to the door, only to find Romeo, who says that he needs their help. But Catboy refuses to help him due to the diabolical cruelty to the PJ Masks and their friends in the past, and also trying to take over the world a million times.
Just then, Captain Jake runs to the door, greets Romeo, and then pulls Catboy aside. He tells him that just because Romeo has tried to take over the world a million times, doesn't mean they can't help him find the ingredients to make the medicine Doctor Undergear needs for his flu. And he also reminds Catboy that a hero helps anyone, even nighttime villains. Catboy reluctantly agrees, and he returns to Romeo, telling him that he and his friends will help if he promises to not make anymore inventions to take over the world. Romeo groans and begrudgingly hands Catboy the list of the medicine's ingredients, which he accepts with a grin.
After the rest of Captain Jake's crew and the PJ Masks agreed to help Romeo collect the medicine's ingredients, the crew went in the Mighty Colossus while the PJ Masks went into the Gekko Mobile and Romeo went to his Shark Ship to go to there first location. Later, they arrived in Tiki Forest to find the first ingredient: the sweetest sap of the Tiki Tree, and the only way to get it is by asking the Tiki Trees. But Romeo rudely demands for the sap, which offends the Tiki Trees that they refused to give him the sap.
Luckily, Captain Jake helped get the sap from the Tiki Trees by asking nicely. That was one ingredient done, only five more to go, as Catboy crossed the first ingredient off the list. The next ingredient was sparkly sea kelp from the Mermaid Lagoon. However, Romeo was having trouble pulling the kelp out of the sea ground just as Gekko helped him pull it out and gave the sparkly sea kelp to him. Groaning a thanks, Romeo snatched the kelp out of Gekko's hand and Catboy crosses the second ingredient off the list as Captain Jake, Izzy, Cubby, Skully, the PJ Masks, and Romeo went off to the Neverland Desert after saying goodbye to the mermaids. Romeo thought that he saw the mermaids cast a scowl at him as they left.
The next third ingredient in the Neverland Desert was the Never Nile water from the Never Nile. Luckily, it was easy enough to get the water as Romeo filled an empty bottle full of it. Now for the next ingredient: five fairy pink roses from Pixie Hollow. As they arrived into Pixie Hollow, the gang and Romeo finds the fairies of Pixie Hollow having a party.