The 19th century was a realm brimming with immortality and incorporeal entities such as vampires, golems, and werewolves. In the midst of this enigmatic world, Aya Rindo, a breathtaking immortal, and her devoted attendant, Shizuku Hasei, embark on a journey with Tsugaru Shunichi - also known as the "Demon Killer" - as they tread through Europe. As "The Cage User," an otherworldly detective, they traverse through supernatural conundrums, on a quest to recover Aya's corporeal form.