At one of Cosette's opera performances, Louis waited outside to see her. When she came out, Louis pined to see her, but Valdemar, her vocal mentor, led her into a coach without her seeing anyone. The Eleventh Doctor and Amy Pond soon caught up to Louis and they had a chat over coffee. Later, they made a plan and broke into Monsieur Valdemar's house, but Valdemar had Cosette sing/scream them into unconsciousness.
Later, Louis held Cosette to prevent her from following Valdemar. He figured out that Valdemar was his great-great-grandson, and so he threatened to drink poison so that Valdemar could never have existed.
Later, Louis said goodbye to the Doctor and Amy, but couldn't find Cosette when the TARDIS dematerialised. (COMIC: The Screams of Death) Chiyoko later returned Cosette to him. (COMIC: The Child of Time)