Kazuya walks into the library looking for ghost stories and finds Victorique instead, a brilliant girl locked in a tower of her own making, and from that moment the show becomes about two broken people solving mysteries that don't matter nearly as much as the fact that they're solving them together. The early cases feel like parlor tricks, but that's deliberate, the show is waiting for you to stop caring about who the culprit is and start caring about what happens when Victorique's past catches up with both of them. By the final arc, every mystery you thought was separate collapses into one brutal truth, and you realize the real case was always about whether these two people could survive knowing each other at all.

Kazuya arrives at Saint Marguerite Academy as an outsider, his dark features earning him an ominous nickname among the student body. While exploring the school's grand library in search of ghost stories, he spots a golden strand of hair on the stairs and follows it to a large garden, where he meets Victorique, an enigmatic girl who skips class and spends her days reading. She has a gift for cracking cases that even detectives cannot solve. What begins as casual curiosity transforms into something deeper as Kazuya finds himself drawn into Victorique's investigations. New puzzles arrive quickly, among them a ghost ship and an alchemist said to wield the power of transmutation, and the two come to depend on each other. The story blends elaborate mysteries with drama and a developing romance, set in a small European country in 1924.