On a raining evening, middle-school student Yeo-Seon (Kim Sae-Ron), resident of a middle-class apartment complex, walks home alone from the bus stop, but she doesn't make it home. 10 days later, her decapitated body is found in a red suitcase. Gyung-Hee (Kim Yunjin) is Yeo-Seon's stepmother. She carries tremendous guilt, because she did not pick her up stepdaughter on the evening that she was abducted. A few days later, Gyung-Hee is so startled, she falls to the floor in the parking lot of her apartment complex. A girl stands in front of her. The young girl's name is Soo-Yeo (Kim Sae-Ron) and she carries an uncanny resemblance to her murdered stepdaughter. Soo-Yeo also lives in the same apartment complex. Another 10 days later, the murderer has yet to be caught and he has killed again. Meanwhile, several people in the apartment complex begin to suspect one of their own residents...