Yamada Yuuki is an ordinary college student with an extraordinary the classical martial art of Kukishin-ryu. Until one fateful day when a demon rips through the fabric of space-time, abducts everyone in his dojo, and transports them to another world. To return home, Yamada and his friends must join forces with other displaced humans to conquer the dungeon that runs through the heart of the world. Standing in their way are endless hordes of bloodthirsty monsters and countless traps. Armed only with steel, faith and guts, they must battle their way through the winding catacombs to confront the demon waiting at the bottom floor. Yamada was once a student. Now he must become a samurai.

Six months after being spirited away to a land filled with deadly monsters, Yamada Yuuki and Hiroshi Matsuo are about to face their toughest challenge yet. They and their fellow samurai must dive deeper into the world-spanning dungeon, going where no man has gone before, in a quest to confront the demon who brought them to its realm. But as the samurai grow in skill and power, so too do the monsters. Legions of bloodthirsty beasts stalk the shadow-filled halls of the labyrinth, far more dangerous than those on the upper floors. Strange spells echo in the corridors, visiting death and madness to the unwary and the unfortunate. An ocean of agony and suffering lies between the humans and the bottom floor. To stand fast against the coming onslaught, it is no longer enough for Yamada and Hiroshi to serve as samurai. They must don the mantle of the Kami no Kishi: the Knights of God.

The war for the dungeon reaches its climax! A hero has fallen. The threat of famine looms over humanity. The endless hordes of monsters inhabiting the world have grown even more powerful. The human military is at the breaking point. All hopes rest on the shoulders of Yamada Yuuki, a college student turned battle-hardened samurai. Armed with faith and steel, experimental weapons and new tactics, he must lead the way into the depths of the dungeon through swarms of horrific abominations and countless traps. But the dungeon itself is changing. And at the bottom floor, the demon ruler of the world awaits. The final campaign begins. A desperate, gruelling crusade to win through the dungeon and find a way home. Now is the time for Seisen. Now is the time for holy war.