Psychology professor Lin Xia found a strange girl in the group photo at a ten-year party of her college best friends. Everyone insisted that her name was "Chen Yu" and that she was their inseparable best friend, but there was no such person in Lin Xia's memory. As the investigation deepens, Lin Xia discovers that this "non-existent best friend" is closely related to her childhood trauma, family secrets, and even an unsolved case.
