Gabimaru reigns as the strongest and most ruthless assassin in his village. But now finds himself on death row—with only one way out: retrieve the Elixir of Life from a sinister island. Longing for freedom, he accepts the challenge. But with fellow convicts vying for the same prize and demonic beasts lurking, how will Gabimaru survive this harrowing quest?

Ganta Nakami is a high school student who suffers from insomnia. He meets Isaki Magari, a girl with the same condition. A relationship forms as they share a secret and catch up on their sleep in their school’s abandoned observatory.