Kimizuka, a middle-school student, meets the beautiful detective Siesta in the process of thwarting a plane hijacking. As they next investigate a mystery at Kimizuka’s school Culture Festival, Siesta urges Kimizuka to become her sidekick.

Kimizuka is approached by Nagisa Natsunagi, a girl with a yearning seemingly inspired by the heart she received in a transplant. When Kimizuka asks Fuubi for help, she takes them to meet an unexpected someone who provides surprising information.

Siesta and Kimizuka investigate a series of murders by someone called Cerberus, who for some sinister reason steals victims’ hearts. As Cerberus eludes Siesta’s pursuit, she comes up with a more desperate tactic, much to Kimizuka’s dismay.

In an attempt to rescue Alicia, Siesta, Kimizuka, and Char set off to confront Hel and SPES on their home ground. There, they encounter one alarming surprise after another and come to realize that with SPES, nothing is as it seems.

Siesta, returned in spirit, partners with Kimizuka in a desperate battle against an ever-changing and resilient foe. In the aftermath of this battle, the survivors carry lingering questions and doubts, and must try to find closure amongst themselves.