During the Northern Wei Dynasty, Mulan joined the army for his father and returned with honor. Ten years later, Rouran broke the border again, and Mulan resolutely returned to the battlefield.

The story starts because of one journey. Seven people who have never met before climb aboard the same ride that goes into a seemingly endless tunnel. Despite the countless trials, corpses that mysteriously appear, past acquaintances that returned from the dead and an unseen danger threatening to swallow them whole, it's as if they are trapped in an enclosed space where there is no escape.