This side-story prequel to Immortal Rain follows Zol, a bounty hunter who works under the name Grim Reaper, during his first encounter with Rain Jewlitt. Rain is better known as Methuselah: the most wanted man in the world, and one who cannot be killed no matter how many times he's shot. The gaiden also traces how Machika came to be Zol's granddaughter and the death of her mother, laying groundwork for the pursuit that drives the main series.