While having to go without Tikki, Marinette must stop Sandboy, a supervillain bringing the citizens' nightmares to life.

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.