Crisis-prone high schooler Kimihiko Kimizuka gets dragged into detective work by the silver-haired Siesta, and the two spend three years crossing the globe fighting criminals and averting disasters before she dies. Back home and trying for a normal life, Kimihiko runs into Nagisa Natsunagi, a girl who looks unsettlingly like his late partner, while Charlotte Arisaka Anderson and Yui Saikawa complicate things further. This light novel is the original source for the manga and anime.