There once was a land where humans and monsters both lived. The sun protected the humans during the day, but at night the monsters were on the prowl. On the day that the sun disappeared forever behind the clouds, the terrifying monsters, no longer warded off by the sunlight, hunted the humans in the day and the night.

For almost thirty years, Saber and his village were cut off from the rest of the world. They have no choice but to communicate with each other using sign language to avoid the monsters, who can track them by sound and especially by voice. However, the village encounters trouble when a young woman named Lune arrives in the village. She tells them that there is a better way to fight monsters--by becoming monsters themselves.

This is only the beginning of a harrowing and thrilling adventure. The villagers leave everything behind, traversing the looming mountains that surround them to find other humans who can help them. In order to escape a life devoid of peace, it is up to Lune and the villagers to summon something they have long forgotten--the courage to live.