A new villain is hatched from a strange crystal on the moon.
It’s harvest moon again, which means Luna Girl can travel to the moon. She summons a moon bubble, but Motsuki hijacks it and flies off to the moon without her. The other moths realize what is going on, and direct Luna Girl to the museum. There, she finds an exhibit with moon rocks, decorated with carvings that depict a moth transforming into a girl. Luna Girl realizes this will happen to Motsuki, and it thrilled at the thought of getting the little sister she always wanted. However, the crystal needs help hatching. And Luna Girl intends to use the PJ Masks for that purpose.
While Motsuki arrives on the moon, Amaya, Greg and Connor are observing the harvest moon from Amaya’s house. They notice the strange glow on the moon, and decide to investigate. Greg isn’t too thrilled at the idea of another moon mission. When Amaya’s telescope almost falls out the window, Connor and Amaya blame Greg, thinking he’s nervous. Greg however denies it was his doing.
The three friends transform and head for HQ. Luna Girl follows them there. It turns out she and her moths were behind the telescope incident. Now, she uses her magnet to force the door of HQ open and let some moths inside. One launches HQ, and makes it look like Gekko did it. This sparks another argument between the heroes, which is just what Luna Girl wants since the bad energy allows Motsuki to transform.
Once on the moon, the PJ Masks explore the surface on their rovers. But due to the moths Gekko’s rover goes out of control and slams into Owlette’s, causing her to fall off her rover, which takes off on its own. Meanwhile, Luna Girl arrives at her fortress and begins to prepare it for Motsuki’s arrival, including new prisons the two of them can trap the PJ Masks in.
The PJ Masks find the cave where Motsuki is transforming. Catboy wants to sneak in, but Owlette’s rover (which is still riding without a driver) causes him to fall in. Gekko and Owlette follow. They see the crystal with Motsuki, but it begins to shoot energy beams at them. The heroes seek cover behind a rock. Two moths move the rock a little so Owlette gets hit by the beams and is trapped in a forcefield. Catboy claims Gekko moved the rock, but Gekko denies. Catboy tries to distract Motsuki while Gekko frees Owlette, but this plan fails and all three heroes end up captured.
Luna Girl arrives, and confesses her whole plan, including how she caused all the arguments between the PJ Masks. The crystal hatches, and from it emerges Motsuki; now transformed into a new humanoid form and with the ability to speak. Luna greets her new sister and suggests they take the PJ Masks to the Lunar Fortress to drain their powers. However, Motsuki no longer wants to obey Luna Girl, causing an argument between the two.
With the villains distracted, the PJ Masks use the shards of Motsuki’s crystal to destroy the forcefields. The heroes flee back to HQ and take off, with Motsuki and Luna Girl in pursuit. The two villains combine their powers to attack HQ. Gekko and Owlette both leave HQ and challenge Motsuki and Luna to zap them. This causes another argument between the villains, who can’t decide who to attack first. They struggle for control of the Luna Magnet, and thus end up breaking the moon bubble they were in.
The PJ Masks can now continue back to Earth without further trouble, but have to live with the fact that they helped a new villain to hatch. Catboy is convinced they can find a way to stop her. The heroes shout their victory cry.