When Irene was younger, she had lost something important. Not only that, she had also lost her voice turning her mute and her will to live had vanished. She didn't care about anything, even herself. Seulgi is a happy-go-lucky person,everything is bright when she's around , she would always be there for people in needed. She felt that there's no one in the world that shouldn't be helped. 

Since no one can communicate with Irene, they turned her into a human doll that's only there to fill the classroom. What if Seulgi's the one that can communicate and possibly pull her from the darkness to their own world. Different yet one thing connects them.