At a ten-year reunion with her college best friends, psychology professor Lin Xia notices an unfamiliar girl in their group photo. Everyone insists the girl is called “Chen Yu” and was once their inseparable friend, but Lin Xia has no memory of her at all. As Lin Xia investigates, she discovers that this “nonexistent friend” is closely connected to her childhood trauma, her family's secrets, and an unsolved case.
