Middle school student Sang-min and his nine-year-old sister Yang-hee are waiting for their mother to come home from work. Sang-min calls his mother, but she doesn't answer the phone. Sang-min's father notices Sang-min's call and rushes to meet his children for the first time in three years, with his wife locked in the trunk of her car. As Yang-hee and Sang-min prepare dinner, the doorbell rings. Sang-min remembers his mother's instructions: "Always check to see if someone is at the door before you open it." She said this just a few days ago, her face filled with worry. When Sangmin looks at his father, he understands the meaning of those words.