It's the 13th century, and the flames of war have engulfed East Asia, and Genghis Khan is set to conquer the world. In the Mongol Empire's latest conquest, the Tangut people find themselves the target. With the recent defeat of Shut Hell—a notoriously vicious female Tangut warrior—the invasion is set to be quick and easy. This conquest stands apart from others however; the objective is not to simply seize the land, but to utterly destroy the Tangut culture and erase them entirely from history.

In present day Japan, Sudou is a high school slacker who is plagued by vivid dreams of ancient times. One day, he meets a sullen transfer student named Suzuki who shows an odd interest in him. However, their encounter is set to vastly alter his life. When Suzuki cries out "Shut Hell," Sudou finds his world slowly fading to black.

Awakening at an abandoned gallows, Sudou finds himself centuries in the past, now inhabiting Shut Hell's body. He is soon discovered by Yurul, a young boy who resembles Suzuki and possesses a complex relationship to the Mongols and Shut Hell. Realizing that "she" doesn't remember anything, Yurul begins to recount the tale of how they met and their current situation—one which makes them the target of the world's most ferocious empire.