After Gabriel Agreste sacrifices his life to remake reality and heal Nathalie, Marinette makes the momentous decision to lie to Adrien and the world. She frames Gabriel as a fallen hero who aided in defeating Monarch, burying his true identity as the supervillain.
Unbeknownst to Marinette, Cerise (Lila Rossi under a new identity) acquires the Butterfly Miraculous. Cerise transforms into the temporal supervillain Timestalker and travels back through key historical events to discover Ladybug’s true identity.
Because de-akumatization erases a villain’s memory, Cerise develops a clever workaround: right before detransforming, she records Ladybug's identity as "Marinette Dupain-Cheng" in a notebook. Reading her own notes upon reverting to human form, she retains the knowledge, steals the Ladybug and Cat Miraculouses in an alternate future, and makes a universe-erasing wish.
In the Burrow, Bunnyx watches time portals vanish as the future erases. She rescues Marinette seconds before her timeline dissolves, granting her temporal powers to become Chronobug.
Together, Chronobug and Bunnyx travel back through temporal anchors—including the Origins timeline and the Agreste crypt—confronting Timestalker across multiple eras. Chronobug manages to de-akumatize Timestalker before she can write the truth in her notebook, neutralizing the paradox and restoring the proper future timeline.
Back in the present, Nathalie overhears Marinette conversing with Tikki and Plagg. Realizing Marinette is Ladybug and understanding the heavy burden she carries regarding Gabriel’s legacy, Nathalie pledges her support, providing Marinette with a trusted adult confidante. The film concludes with Marinette reuniting with Adrien at the train station and a seamless visual transition into Season 6.