Han Si-jun, a field agent of the National Intelligence Service, goes undercover as a priest at the Russian St. Sergius Cathedral. He meets Anna, a Korean orphan, at the cathedral, and their relationship leads to his first love. However, whenever he looks into Anna's eyes, he feels a strange sense of foreboding, which gradually brings back old memories that he had forgotten. And apart from his innocent first love, the target of this operation is gradually revealed from the depths. In order to stop the illegal arms trade of the Russian mafia and protect Anna, Si-jun must now face his past self.