In Korean folk traditions, "seire" is the 21 days after a baby is born in which they're uniquely vulnerable to bad luck, curses - even evil spirits. Many superstitions surround these first three weeks in a baby's life, and new mother Hae-mi (Sun-young Ryu) carefully follows every one of them. Father Woo-jin (Seo Hyun-woo) is reluctantly willing to play along, but when he attends an ex-girlfriend's funeral, he unwittingly opens the door to dark supernatural dangers. Plagued by terrible nightmares, the skeptical Woo-jin begins to understand the seire custom, as he is forced to unravel the mystery of what is haunting him and his family.