Goblin Slayer and his party receive a request to slay some goblins. The one who sent the request was the Sword Maiden, one of the heroes that defeated the Demon Lord. Goblin Slayer, Priestess, High Elf Archer, Dwarf Shaman, and Lizardman Priest head to Water Town. It's a peaceful, beautiful city, but apparently, there are some scary incidents happening. The party believes that there might be some goblins hiding in the sewers so they head down there as soon as possible.

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...