When Tikki tries to stop Plagg from eating a piece of cheese in the classroom, Ms. Mendeleiev, Collège Françoise-Dupont's physics teacher, catches them, but they escape. Ms. Mendeleiev tries to prove the existence of the magical creatures on a TV show, but the viewers are not convinced by her theory. Marinette and Adrien become convinced that their partner must be one of their fellow students, and Adrien even specifically suspects Marinette; conversation reveals that they must give up their Miraculouses if their identities are compromised, even to each other. Angry that no one believes her theories, Ms. Mendeleiev gets akumatized into "Kwamibuster", a Ghostbusters-themed villain with a vacuum to capture kwamis. Both superheroes lose their kwamis to Kwamibuster, so Adrien hides in a costume, and Marinette flees to Master Fu. She borrows many other Miraculouses despite the physical risk, and has Mullo of the Mouse Miraculous transform her into "Multimouse", giving her the power to divide herself into miniature clones. Some of her clones also use "unify" to undergo alternate transformations and employ the various kwamis to best Kwamibuster, freeing all the captured kwamis. Multimouse then uses the Fox Miraculous to make it appear that Marinette and Ladybug are two different people to erase any idea from Cat Noir of them being the same person. Cat Noir apologizes to Ladybug for his kwami being at "that elementary school" (to convince her that he is not a student there) and discourages Ms. Mendeleiev from making any more attempts to prove the kwamis are real. Marinette returns all of the extra Miraculouses to Master Fu. He remarks how exceptionally strong Marinette is to have used so many at once.

Hawk Moth akumatizes Chloé into Miracle Queen to make her take control of all Miraculous holders. Will Ladybug and Cat Noir be able to escape this fatal plan?

Since becoming the new guardian of the Miracle Box, Marinette has been overwhelmed. She doesn't see her friends anymore, abandons Cat Noir, and misses all her dates with Luka. Luka senses that Marinette has a secret, but she can't tell him that when she leaves, it is to become Ladybug and save Paris. Deeply hurt that Marinette doesn't trust him enough to confide her secret, Luka is akumatized into Truth. Helped by his sentimonster, Pharo, a giant eye capable of immobilizing people so that way Truth can hit people with his beam to tell the truth, he not only wants to know Marinette's secret but also Ladybug and Cat Noir's secret for Shadow Moth.