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.

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.