Rin Okumura watches his twin Yukio defect to the Illuminati in pursuit of answers about their demonic heritage, and resolves to uncover the truth himself. Mephisto Pheles gives Rin a key that sends him decades into the past, where he witnesses his mother Yuri Egin's path from exorcist trainee to the woman at the center of the Blue Night — the catastrophe that brought Satan's flames into the human world and led to his own birth.