The story takes place in a dystopian city known as "No.6". Shion, a boy raised in the elite and privileged environment of his home, gives shelter to another boy, who only gives his name as "Nezumi" (or "Rat") on the former's twelfth birthday. What Shion soon discovers is how drastically life will change after meeting with the mysterious Nezumi, with whom he had shared one unforgettable, stormy night.

First published in Japan in 2011 by Kodansha Ltd., Tokyo. Reads manga-style, right to left and back to front.