Born from the corpses of fallen soldiers, the "Undying" are near-immortal beings created for the sole purpose of being weapons. However, after the War came to an end, so did their usefulness, and the Church began slaughtering them mercilessly in the Undying Hunt. Eighty years have passed since then; there are now barely any remnants of the Undying, and those found alive are hunted down by the Church.

Kieli, an orphan studying at a religious boarding school, possesses the unique ability to see spirits. Isolated from her classmates, she spends her days talking with Becca, the spirit of a girl who used to inhabit her room. One day, Kieli runs into an aloof and mysterious individual named Harvey, one of the few surviving Undying. Following an unexpected turn of events, Harvey saves Kieli from being run over by a train, putting himself at risk in the process. Wanting to learn more about Harvey, Kieli decides to accompany him in fulfilling his promise to the Corporal, the spirit of a soldier who died in the War and has since been possessing a radio—the promise to reunite him with his mortal remains.

Throughout their journey, Kieli discovers that beyond Harvey's seemingly ruthless nature lies a kind and thoughtful man tormented by the horrors of war. As the two begin to interact and grow closer, they will develop a bond that they can forever cherish, and Harvey may finally find the warmth that he once lost.