During the process of relocating, Ryuuhei, a pupil in elementary school, stumbles upon his father's former school notes which describe the hunt for a prized possession located in Tokyo's extensive network of underground passages. Ryuuhei, accompanied by his younger sibling, joins forces with three acquaintances met through an online chat room, to embark on the search for the treasure. Ultimately, their journey leads them to uncover an entire subterranean civilization, made up of diverse personalities, which includes an elderly war veteran persisting in his assignment from the Second World War despite his declining mental faculties.