When Mu Yueli and Mu Yan met for the first time, Mu Yueli was six years old and Mu Yan was eleven years old.

Mu Yueli can't forget the scene when she and Mu Yan met for the first time in her life, because from that moment on, she was like opening Pandora's box and spied on the secrets of the Mu family and the cause of death of her parents who had been dead for many years.

In Mu Yueli's memories before she was six years old, the world was a room of less than 20 square meters - it was surrounded by walls, no windows, no sunlight, and the only furnishings in the room were a bed, a table, a chair, a wardrobe, and no electrical equipment other than electric lights.

When she was seven years old, her grandfather led her out of the house, and she realized that it was called the basement.

At that time, a German nun lived with Mu Yueli and took care of her daily life. The German nun could only speak German, and she couldn't understand her in the month, and every time she wanted to tell her something, the nun always looked at her with a puzzled face, and after a few seconds, she went about her own business. Six-year-old Mu Yueli stood barefoot, dragging dolls, behind the busy nun, and suddenly felt as if the world was only left with herself, and she understood what it meant to be lonely.

The nun would leave the house regularly every week, for about half a day, which was the hardest time in the month of Mu. Every time the nun went out, Mu Yueli locked herself in the closet, closed her eyes, counted from one to a hundred, and then from one hundred to one, and so on until the nun returned.

When the nun returned, Mu Yueli was the happiest, because she always brought back some picture books, a few new clothes, fresh ingredients and a few packets of delicious snacks.

On the night the nun returned, Mu Yueli always ate a slightly more hearty meal than usual, and she often wondered what the "outside" the nun was like every time she went. It must be bigger than this room, right? Is it a castle, as depicted in a storybook? Have an endless meadow or garden? Is there also light outside, but not from electric lights, but brighter, more expansive? The kind of light that is called the sun?

Mu Yueli could only guess, or sketch in a dream, she couldn't get out of here.

Once, she tried to gesture to the nun and said that she wanted to go "outside" with her, but the nun always shook her head. When Grandpa came to see her every few months, she also expressed to this somewhat strange relative, the idea of wanting to go out, but Grandpa's expression always became extremely heavy, and his hand on crutches trembled slightly, and then, unsurprisingly, resolutely rejected her.

It's impossible to leave here from regular channels. Mu Yueli angrily crossed out the names of nuns and grandfathers on her "escape plan".

How do you get out of here? Xiao Yueli lay on his "escape plan" book and racked his brains. Suddenly, a crescent moon appeared on her face, and she immediately sat upright, happily picked up the pen, and wrote a line crookedly on the "escape plan" - follow the nun and find the opportunity.

The nun went out at a fixed time every week, and before she went out, she would always lock Yueli in her hut, and use gestures to warn her not to go out, and wait here for her to return.

Inside the nun's suitcase, there was a Bible with a spare key to the room. It was when the nun accidentally cut her hand in the moon, and when the nun was looking for a band-aid, she inadvertently fell to the ground. Thinking that Moon would not notice, the nun calmly put the Bible back in the box. She didn't notice that the eyes behind her sparkled as if they had found a treasure.

When she goes to wash Tsukisato's blood-stained shirt, Tsukiri slips the key into her socks.