Rin Okumura embarks on a quest to uncover the truth about his origins as the son of Satan. Guided by Mephisto, he delves into the past, seeking answers about his adoptive father, Shiro Fujimoto, and their mother, Yuri Egin. This journey unveils the harsh realities of their lineage and the tragic events known as the Blue Night, setting the stage for Rin's internal conflict and growth.