Sister Yunia (portrayed by Song Hye-kyo) crosses paths with Hee-jun (played by Moon Yoo-jin), who has been possessed by one of the Twelve Demonic Spirits, during an unforeseen incident. With the exorcist priest failing to arrive in time to assist, the malevolent spirit's power grows increasingly overwhelming. In a desperate, life-threatening moment, Yunia is forced to break the sacred taboo that forbids nuns from performing exorcisms. Teaming up with Sister Michaela (played by Jeon Yoo-yoon), the two join forces, determined to save Hee-jun at all costs. Their mission pushes them to the brink as they resort to using one of the most perilous and forbidden exorcism rites. As the ominous presence of the spirit looms, can the sisters fight fire with fire and ultimately triumph over the darkness?