The episode begins with the four PJ Masks in the Cat Car chasing Romeo who has stolen a special and magical gear from the Disney Junior Town Museum. When they finally caught up to him, they demand Romeo to hand over the cog but he refuses as he explains to them that he'll use the gear for another invention that he will use to take over the world. Angered and annoyed, Fish Boy immediately tells Romeo that he'll never take over the world with the gear that big. Romeo ignores him and says that he will, and tells them that everyone doesn't call him Romeo Undergear for nothing!
Fish Boy hears the last name "Undergear" and asks Romeo if he is related to his arch enemy, Doctor Undergear. Hearing Doctor Undergear's name, Romeo nervously snaps at the four PJ Masks and then throws the gear as he leaves embarrassingly. Before the three PJ Masks could shout hooray, Fish Boy stops them and suggests that they should follow Romeo to find out if he is an Undergear or not, even though Catboy secretly disagrees.
Back at the mansion, which was where Romeo must live in, Romeo secretly parks his lab and sneaks back up into his bedroom.
At Romeo's bedroom window, Fish Boy, Cat Boy, Owlette, and Gekko, are watching Romeo in his room. There, they notice that Romeo has a room that looks like an evil inventor's workshop, blueprints and sketches of his latest inventions, scraps of metal, a half sub and half boat shark, two giant mechanical crabs and a robotic bird like the ones at Gear Island as pets. Catboy then says that it confirms that Romeo is related to Doctor Undergear and leaves but Fish Boy stops him and tells them that they should listen to Romeo first to find out if there is anything that can prove that Romeo is related to Doctor Undergear.
Inside Romeo's room, Romeo tells his robotic pets that his life is so miserable and wishes that his grandfather was here, if he hadn't left. Then his mechanical pets gave him a hug, and he turns to his pets with a smile and says that he is so lucky to have his grandfather invent them for him to keep him company and also pulls out a gold chain with a golden cog-shaped pendent with a picture taped on it to remind Romeo of his grandfather. As the PJ Masks drew closer, they soon discover that on the picture was him smiling and sitting on the lap of... Doctor Undergear!
Gasping about their discovery, Fish Boy decides that he and his friends should camp out and wait until morning comes so Romeo, as his daytime self, can come out for school and that they should play nice with him.
The next morning, the PJ Masks (now back in their daytime selves) watches Romeo outside in a blue tent. Captain Jake takes out his binoculars to see that Romeo's parents will not take him to the Robotics Expo because they have to work at the observatory. Romeo gets teary eyed and walks off, holding his lunchbox and carrying his Robo Bird on his arm, but runs into Connor, Amaya, Greg, and Captain Jake (who managed to hide the tent just in time) and asks them to walk to school with him because he feels lonely walking to school from home and back again. They quickly agreed and walked with Romeo.
Later at Disney Junior Elementary School, Captain Jake, Connor, Amaya, and Greg decide to eat lunch with Romeo instead of eating at their usual lunch table with their other friends. While drinking his milk, Greg notices the same cog pendant with the picture hanging from Romeo's chain. He compliments it as Romeo holds it up and tells them that his grandfather gave it to him to remind him that he'll always be with him no matter how far he is. But when he puts it back inside his T-shirt, Romeo frowns and wishes that he would see his grandfather again personally because tonight's his birthday as he gets up to throw away his lunch scraps and leaves for the next class miserably.
After he left, Captain Jake, Amaya, Greg, and even Connor suddenly felt sorry for Romeo and decides to do something to cheer him up. Greg thinks that they should take Romeo to Never Land but Captain Jake suggests that if he and the PJ Masks can't bring Romeo to Never Land, then they can maybe bring Doctor Undergear to Romeo.
Later, Captain Jake tricks Doctor Undergear to come to Romeo's house by texting him a message about Romeo's birthday. As Doctor Undergear arrives and knocks the door, Romeo's father opens it and welcomes him inside just. Then he calls Romeo to come down. Romeo, with his robo bird on his shoulder, and one of his giant pet crabs, comes down the stairs to see his grandfather for real. But instead of going over to Doctor Undergear to hug him, Romeo scowls at him and runs back upstairs until his mother stops him.
Doctor Undergear asks why Romeo wasn't happy to see him but Romeo was already turning away back upstairs to his room, and then slams the door angrily. After his parents apologized to Doctor Undergear, the sad mad scientist walks away when Captain Jake, Connor, Amaya, and Greg popped out from a nearby bush to see that their plan hadn't worked as they hoped. So forming a pact, they decide to get Romeo and Doctor Undergear together tonight at the PJ Masks! Later when night came in, the PJ Masks transformed and took the Cat Car with the birthday decorations in the trunk and they drove off to the plaza to decorate it.
After they finished, Catboy takes out his iDisney to text Romeo and Doctor Undergear to meet at the plaza. Later, Doctor Undergear arrives to find the whole plaza decorated for his grandson's birthday and Romeo drives his lab to the same location of where is party is. When he got out, Romeo finds himself face-to-face with his grandfather again while the PJ Masks hopefully watched their plan work in action. But like this morning, Romeo was still angry at his grandfather as Doctor Undergear tries to reason with him but Romeo asked him why he had to leave him.
Before Doctor Undergear could explain, Romeo turned away and stomped back towards his lab. As he drove his lab away from the plaza, Romeo puts his hand under his coat and sadly pulls out his cog pendent to take another look at his and Doctor Undergear's picture for the last time, then there was trouble Romeo was so busy looking at his pendant, that little did he know that he was driving straight towards the dock. And before he could hit the break, Romeo's lab went down the ocean with a big splash!
Romeo manages to open the door and swam to a huge rock to stay up on the surface, but then, a pack of never-sharks swam towards him as they surrounded Romeo and stared at him with hungry eyes! Meanwhile, Doctor Undergear felt miserable cause his grandson was angry at him, just then Catboy heard Romeo's cry for help and he and his friends went into the Cat Car to drive to the dock only to find Romeo stranded on a rock, surrounded by hungry never-sharks.
When Romeo saw them, he yells at them and demands to save him just when one of the sharks snapped his jaws at him and he backed away while trying to stay in the middle. Just then, Doctor Undergear in his shark armor dives into the water and swims towards the rock to save his grandson! Doctor Undergear got the hungry sharks attention and soon, they started chasing him underwater which gave Fish Boy enough time to rescue Romeo and take him back to the dock for safety. Meanwhile, under the sea, Doctor Undergear was still being chased by the never-sharks just when he saw a sunken ship to trap them in there and keep them there until they are too exhausted to chase him.
Later, Doctor Undergear swims back to the dock to see that the PJ Masks have saved Romeo. Feeling relieved, Doctor Undergear then stretched his arms out and waited to get a hug but Romeo turned away and was about to walk back to his lab when the PJ Masks stop him and ask him why he wasn't happy to see his grandfather.
Clutching his fists and trying his best to fight back the tears in his eyes, Romeo points a finger at Doctor Undergear and tells the PJ Masks that it's all his fault! For leaving him alone and not coming back in time for his birthday party last year because he was always so busy at Gear Island inventing his own gizmos and not make his birthday present. Fish Boy tried to calm Romeo down, but he was still fuming as he kept on blaming Doctor Undergear.
Just as Catboy was about to respond, Doctor Undergear steps in and finally tells Romeo the truth; he still remembered his birthday but didn't have enough time to finish the present because Captain Jake and his crew kept on getting in his way and the worst part was that Captain Jake ruined his workshop. Romeo calms down, seeming to believe his grandfather's truth.
And with that Romeo gave Doctor Undergear a big hug, so that he'll remember it forever. After that, it was time to party! And everyone had an awesome time, including Romeo, Doctor Undergear, and their mechanical pets. Everyone agreed that it was the best party ever.
- This episode is all about Romeo and Doctor Undergear.
- Two scenes are based on The Secret Origin of Denzel Crocker from Fairly Odd Parents and Talkin Trash from Captain Planet and the Planeteers.
- In a deleted scene Romeo and Doctor Undergear sing a parody version of "This Kitchen's Not the Same Without You" from Spongebob Squarepants.
- This marks the debut of Romeo's Shark Ship and Crab Bots including his Robo Bird.
Captain Jake (Narrating): Romeo Undergear.
Romeo: (evil laughter)
Gekko: Give it back Romeo!
Romeo: Come on, PJ Pests it was just one little gear! (gasps) Whoa!
Catboy: Give up Romeo! Hand over the gear.
Romeo: No way! With this gear, iI will use it for another evil invention and when I do I can finally take over the world in peace.
Fishboy: But you can't take over the world with a gear that big.
Romeo: Yes I can, and I will do it if everyone doesn't call me Romeo Undergear for nothing!
Fishboy: Undergear? Are you related to my arch enemy Doctor Undergear?
Romeo: What? Uhh, no um what are you four losers looking at? Uhh, well it's been awhile doing this. Oh that so embarrassing.
Gekko: Uhh... PJ Masks, I'll shout hooray...
Everyone: Cause in the night we save...
Fishboy: Wait. I think that we should follow Romeo to make sure that he's related to my arch enemy Doctor Undergear.
Catboy: (sighs) Fine.
Romeo: Oh I just can't stand the PJ Pests, but at least I have my pets to talk to.
Gekko: Look! Romeo's bedroom looks like a scientist workshop.
Owlette: There's some blueprints and sketches.
Fishboy: And there's a shark, two giant crabs, and a bird. There like the ones on Gear Island as pets.
Catboy: Well that solves our problem.
Fishboy: Wait! Maybe if we listen to Romeo, we could find out if he's really an Undergear.
Romeo: Oh my pets. My life is miserable. I wish Grandpa was here.
Romeo: Thanks guys. I'm so happy that Granpa invented you so that you can keep me company.