In Akatsuki is the 48th overall episode of Tegami Bachi
Noir points his gun at Garrard and Valentine and asks where the Ones Unable to Become Spirit are. Soon, however, the ground swallows Lag, but Noir saves him. They find an underground lake, which they didn't know was under the Hill of Prayer. Noir then shoots Lag with a shindan, so that Lag would understand that Gauche is no longer part of him and that he isn't Gauche anymore.
While seeing Noir's memories, there is a memory deep inside of him that Noir claims has always been at the bottom of his heart, and that is of a woman crying deeply. Lag finds that it is his mother, and that Gauche truly did fulfill his promise of seeing Lag's mother, Anne Seeing. However, Gauche did convince her to give him a portion of her heart to take back to Lag, she also stole Gauche's heart in the process - mentioning that once he stepped foot into the main part of the Capital where she was, the heart stealing had begun.
As they finish talking and Lag collects himself, they vow together to take down the Cabernet, and since Lag is comfortable with the fact that he is dealing with Noir now since Gauche cannot return, he knows what he has to do to get Gauche's heart back - which appears to not be done by memories alone since Noir was just acting like Gauche when Lag shot the letter bullet into him.