Description：In the humid and noisy metropolis, Yeon-soo (Jung Yu-mi) and her underage sister, Soo-jung (Kim Sae-ron), depend on each other for survival. Soo-jung lost her hearing due to an accident in her early years. The guilty Yeon-soo shoulders the heavy responsibility of raising her sister to adulthood, and takes great pains to take care of Soo-jung's food, clothing, housing and transportation. For a period of time, women mysteriously disappeared in the city, and they were all kidnapped and killed by a man (Jung Kyung-ho) living in the sewer. The man had an unfortunate childhood, and his dark experiences 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 in hunting targets. Until one day, Soo-jung was also targeted by this man. The sudden interruption of the call made Yeon-soo anxious. In order to rescue her sister, she broke into the dangerous place regardless of the danger.