Tantei wa Mou, Shindeiru.

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.

A teen idol, Yui Saikawa, enlists Kimizuka and Natsunagi--who has taken up the mantle of legendary detective--to guard a priceless sapphire. In preparing for this, the two realize that more than just a precious jewel is at risk.

On the night of Yui Saikawa's big stadium show, Kimizuka and Natsunagi struggle to protect her from a hidden assailant. But the further the two dig into the plot against Saikawa, the clearer it becomes that the teen idol has secrets of her own.

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.

Siesta and Kimizuka win the battle with SPES's top executive Hell who suddenly appears. In return, Siesta was injured in her leg for about two weeks. Kimizuka, who was asked to use it, meets Alicia, a girl who was sleeping in a cardboard box in the back alley. Kimizuka asks her relatives, but Alicia has lost her memory so far...

Kimizuka searches for "Sapphire's Eyes" with Alicia, who acted as a detective during Siesta's medical treatment. For a short while, three people visit the dining bar to celebrate the complete recovery of Siesta. Siesta offered Kimizuka a glass to make up for the apple pie, and when he noticed it was late at night. When Siesta returned to the room, she sat down on the bed and called out to Kimizuka, "Come here."

Kimizuka and Siesta realized that the wind that they met the other day was a fake. While upset, a fifth victim was killed. Siesta, who chooses her means to defeat her Hell, and Kimizuka, who wants to respect the feelings of her bereaved family, pass each other. Three days later, Alicia is involved in the incident. She was Kimizuka, who was relieved by Alicia, but she was beginning to realize the unacceptable reality...

To regain Alicia, Siesta, Kimizuka, and Shall finally arrived at the SPES base. Kimizuka, who was split into two hands with Siesta and acted with Shall, confronts SPES's main ball "Seed" and learns the true identity of SPES. Meanwhile, Siesta reunites with Hell and attempts to recapture Alicia.

Saikawa, Natsunagi, and Kimizuka set off on a cruise together, during which Kimizuka unexpectedly encounters a former acquaintance. It turns out that something precious is onboard the ship, something which attracts the wrong sort of attention.

Facing off against Chameleon with Natsunagi's life on the line, Kimizuka finds that he is not alone in this battle, with help coming in unexpected forms. But against this wily foe, Kimizuka's allies--and his own resilience--are tested to the limits.

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.