Kara no Shojo

March 1956.
In a Tokyo that’s slowly returning to its former glory after losing the war ten years prior, Private Detective Reiji Tokisaka takes on the case of a girl he meets in Inokashira Park.
“I want you to find something… Me. My true self.”

A wave of bizarre and grotesque crimes has been crashing down on the city. The victims, all young women, are being kidnapped and murdered, each corpse only to be discovered with a body part and the uterus removed. Sergeant Uozumi, Tokisaka’s childhood friend and former coworker, asks him to investigate the murders.

Meanwhile, sitting off in a corner of the town of Hoya is Oba Girls’ Academy, the school Tokisaka’s younger sister goes to and where two students have recently disappeared under mysterious circumstances. The vice principal of this academy, Tokio Saeki, implores Tokisaka to find them.

Having taken on all three of these cases, and with Vice Principal Tokio’s approval, Tokisaka enters Oba Girls’ Academy as a teacher in order to dig up information about the students.

It’s there that he meets her again. Toko Kuchiki, the girl who asked him to look for her true self, with a voice as light and as clear as a bell and a casual way of speaking that doesn’t match it.
“Heya. Figured I’d be seeing you soon, Detective.”

More and more victims turn up as the days pass, and the investigation he thought would be oh-so-easy stalls. For some reason, the number of disappearances and unidentified bodies don’t add up.

And the newest victim is—

The secrets of a case from six years prior may just be the key to the current unending cycle of tragedy, and the thing that cracks this woeful world’s shell may be one girl’s smile.

From Shiravune's description of The Shell Part I: Inferno