Set in the near future, a block of flats has recently been built in the big city. The block is home to residents from diverse backgrounds, but a new infectious disease breaks out. Due to the outbreak, many floors are sealed off. The residents struggle to survive, whilst gripped by fear and mistrust of one another. Yoon Sae Bom (Han Hyo Joo) is a member of the special forces. She is resolute, determined and does not get flustered easily. She is excited to move into her new flat, but as soon as she moves in, she is faced with chaos. Jung Yi Hyun (Park Hyung Sik) is a detective. He is intelligent and honest. He has harboured romantic feelings for Yoon Sae Bom for a long time. They graduated from the same secondary school. Jung Yi Hyun fights to protect Yoon Sae Bom and others. Han Tae Seok (Jo Woo Jin) is a lieutenant colonel and belongs to the Medical Service Command. He holds the key to the outbreak.