Haunted by the death of her little brother, So-Yeon fights against the vengeful spirits who killed him. She is an Overwatcher: a guardian who pacifies evil and lights the gateway to guide lost souls to peace. Every Overwatcher must protect an Apostle, those with the ability to light the gate. It's said that once an Apostle purges one thousand spirits, they must grant their protector one wish. In a muck of supernatural conspiracies and corruption, can So-Yeon get her wish and find her own.