Beneath The Light is the 10th overall episode of Tegami Bachi
Aria takes Lag and Niche to the Hill of Prayer, where the artificial sun at Akatsuki shines very brightly there. She tells Lag about the Day of Flicker, when a government airship, which was used to make adjustments to the sun, crashed into the vicinity.
Before the crash, the light from the sun went out briefly and flickered back on. This caused Gauche to lose part of his heart and forget about his mother, who died from giving birth to Sylvette. He began to focus all his thoughts toward taking care of his sister.