Goblin Slayer and his party go into the sewers of Water Town to investigate some brutal incidents they think were caused by goblins. When they go down to the sewers, they are immediately attacked by a horde of goblins. Because of the sheer number of goblins, the party has a hard time dealing with them all and then a goblin champion also appears before them...

The party encounters a giant mirror in the sewers. The mirror, the mysterious creature they encountered, the horde of goblins, and the goblin champion that appeared... Goblin Slayer and the others try to get to the bottom of what's going on. They once again encounter the horde of goblins as well as the goblin champion, which gravely injured Goblin Slayer previously. But Goblin Slayer had a plan to take care of them once and for all...