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.

In 'Lies', Kagami thinks that Adrien doesn't love her, and because he's also Cat Noir, he always has to lie to leave her so he can save Paris. Then Kagami finds the lucky charm Marinette gave Adrien, which just adds fuel to her anger.

Ladybug and Cat Noir face someone they know well - and of whom they never thought that one day they would have to fight against him.

After stealing the Miracle Box, Shadow Moth turns into Monarch and uses the Rabbit Miraculous to return to a vulnerable moment in Ladybug and Cat Noir's past to capture their Miraculous.