Kim Jiyoung lives in an apartment in Seoul with her husband Jung Daehyun and their baby daughter Jiwon. Daehyun works in IT. Jiyoung worked in marketing but quit her job shortly before Jiwon was born.

Jiyoung begins exhibiting puzzling behavior on the first morning of autumn. During breakfast, she opens a window, letting in cold air . She recites a rhyme in the voice of an older woman. Daehyun thinks she is doing a humorous impersonation of her mother. That night, when he returns from work, he finds Jiyoung and Jiwon sleeping side by side, both sucking their thumbs.

A few days later, Jiyoung speaks in the voice of her college friend Cha Seungyeon. Seungyeon died in childbirth at a time when Jiyoung was experiencing postpartum depression; Jiyoung was devastated by her friend’s death. When Jiyoung begins speaking as Seungyeon, Daehyun assumes she’s doing another lighthearted impersonation. But she references secret communications between him and Seungyeon that Jiyoung could not know. Daehyun is frightened and confused, wondering if Jiyoung could be possessed. The next morning, Jiyoung has no memory of the strange occurrence. More unsettling incidents follow. Daehyun feels that Jiyoung is becoming a stranger to him.