In the humid and noisy metropolis, Yeon-seo (Jung Yu-mi) and her underage sister Soo-jung (Kim Sae-ron) depend on each other for survival. Soo-jung lost her hearing in an accident in her early years. The guilty Yeon-seo bears the heavy responsibility of raising her sister to adulthood, and takes great care of Soo-jung's food, clothing, housing and transportation. For a period of time, women in the city mysteriously disappeared, and they were all hijacked and killed by a man living in the sewer (Jung Kyung-ho). The man had an unfortunate childhood, and his dark experience distorted his character and soul. He carefully transformed the sewer into an independent kingdom, monitored the whereabouts of passers-by through cameras densely distributed in the streets and alleys, and locked the hunting target. Until one day, Soo-jung was also targeted by this man. The sudden interruption of the call made Yeon-seo anxious. In order to rescue her sister, she broke into the dangerous sewer regardless of the danger...