A man known as Under is immortal, able to regenerate from any wound or death, yet he lives without purpose or memory of his origin. Stranded in the middle of a centuries-long war between humans and demons, he is eventually captured by the Demon Army. After enduring endless torture, including execution by fire, Under loses all will to live.

His fate changes when he is sentenced to Space Execution alongside a fellow prisoner named Recobell. Locked inside a hot air balloon and sent into the upper atmosphere to die, Under begins conversing with Recobell. Through their shared ordeal, Under slowly rediscovers the motivation to live and finds a new goal: to understand the true meaning behind the seemingly pointless war between the two factions.

The setting is a fantasy world torn apart by a grand conflict between humanity and demon forces. Under is established as one of only fifteen confirmed immortals in history. The story follows his journey from an empty shell of a man to someone actively seeking answers about the war and his own existence.

Notable narrative arcs include Under's capture and imprisonment by the demon army, the Space Execution sequence that serves as the catalyst for his transformation, and his subsequent search for purpose while exploring the origins of the human-demon war. The manga was serialized from 2015 to January 2020, and after a hiatus, the series was discontinued by Shogakukan. The author later chose to independently continue and release further volumes as ebooks.