The episode begins as Motsuki spies on Romeo and Robot attempting to rebuild all the inventions that the PJ Masks either destroyed or put in their headquarters. When she hears that they made a portable version of the Baby Beam, she gets an idea and takes it from them. She approaches Luna Girl, who is demanding her to steal things, and zaps her with the invention, intending to make her her little sister so she can do whatever she says.
The next day, all the art supplies are scattered around the school, and some of the art projects are splattered with paint. That night, the PJ Masks see in the PJ Picture Player that Motsuki stole the Baby Beam and turned Luna Girl into Baby Luna. Seeing that she’s painting on more objects besides the canvas, the team heads off to stop her.
In the forest, Motsuki is taking pleasure at telling Luna Girl what to do, although she gets angry at her for throwing a paintbrush at her. The PJ Masks arrive and confront Motsuki, but she traps them into bubbles, and they float away. At the same time, Romeo and Robot arrive at the scene and demand Motsuki to hand back the Baby Beam. The three spot the invention and Robot tries to grab it, but Motsuki zaps him as well, and he is bubbled. He floats away, dropping the Baby Beam, and the impact causes the invention to activate and zap Romeo into a baby as well.
Motsuki proceeds to tell the baby villains what to do. Meanwhile, the PJ Masks float back to headquarters. PJ Robot frees them by zapping the bubble shields and shows them through the tablet that Romeo has also been zapped. Catboy, Owlette, and Gekko follow the villains to the school as they leave more mess behind. They try to confront Motsuki again, but Luna Girl zaps them with the Baby Beam, and they are turned into babies. With everyone now turned into a baby, Motsuki leads them inside the school.
Inside the classroom, the baby heroes and villains are running around and throwing things everywhere. Motsuki grabs their attention with a loud squeak and tells them to listen to her. She tells the group to be bad, and they proceed to do just that. Motsuki, though, is pulled along with their antics. As she is carried outside, the group throws her around, using their powers on her. She demands them to do what she says, but the group continues doing what they are doing.
PJ Robot suddenly arrives in the PJ Seeker and sees all the commotion. Motsuki asks him to help, and he grabs a volleyball for the kids to play with. They drop Motsuki, and they proceed to play catch. Motsuki asks him how did he get the kids to listen, to which he responds that he was nice to them. Motsuki counters this by saying that she is mean, and that it only works for the PJ Masks since they are nice already, but when she sees the team pass the ball to Romeo and Luna Girl, she is taken aback and, although reluctantly, decides to witness how PJ Robot treats the five baby-turned kids.
Everyone returns back to the classroom, and PJ Robot teaches them how to be nice. He gives the PJ Masks art supplies so they can draw, gives Romeo and Luna Girl a drink as they play with the building blocks, and assists the group with whatever they need help with. While the montage is happening, Motsuki slowly realizes that each behavior that a big sibling exhibits will cause the younger siblings to act as she does. Therefore, if she acts nice, then the group will act nice as well. She then follows what PJ Robot did before, and she ends up enjoying the kids’ nice behavior. The babies give her a group hug, and Motsuki tells them that she loves them.
PJ Robot suddenly notices the Baby Beam on the desk. He re-adjusts the setting, but Motsuki zaps him into a bubble shield before he could zap the five babies back to normal, not wanting to become a little sister and have the kids become big again. She grabs the invention as it drops to the floor, but she notices the kids’ reaction. She figures out that the kids won’t be able to grow up, just as she won’t be able to do so herself. With this in mind, she decides to zap the group into big kids again and zap PJ Robot free.
The group is back to normal, unaware of what happened, but when PJ Robot tells the PJ Masks they were bad babies turned good thanks to him and Motsuki, they remember that they were stopping babies Romeo and Luna Girl because they thought they were trouble. Despite that, PJ Robot liked them and knew they could be nice, and the team hopes that they could be that way from now on. However, Romeo and Luna Girl quarrel with each other, and after being meanly treated, Motsuki chases them out of the classroom. Nevertheless, thanks to PJ Robot, there are no more baby villains to fight, and the team shouts out their signature victory phrase.
Teach others how to be nice instead of teaching them how to be bad.
In the classroom, we see Romeo’s terrible drawing of him getting superpowers, which is from "Wolfy Powers."
While Luna Girl and Romeo giggled after being zapped by the Baby Beam, the PJ Masks only cried like babies after the former zapped them with it. As a result, babies Romeo and Luna ended up calming the PJ Babies down.
The PJ Masks' victory quote was given slight alterations to "PJ Masks, all shout hooray! 'Cause in the night, PJ Robot saved the day!"
Starting with this episode, Luna's eye color is now more of a purplish-blue than regular blue.
Luna Girl (as Baby Luna) uses one of Romeo's inventions for the second time.
PJ Robot is called "PJ Bot-Bot" for the first time.
Robot was also called simply "Bot-Bot" for the first time.
This is the last episode to feature Owlette doing her original animation on activating the "Owl Wing Wind" power to have her flapping the owl wings before the producers decided to change the animation to have her fold and release her wings at the last minute before the production of Season 3 ended.