In the year 526, when witches were known to exist but magic was still a mystery, a mercenary of mixed human and beast heritage, scorned as a 'fallen beast' by humans, aspired to shed his animal form and become fully human. His chance for transformation arrives when he encounters a witch named Zero who offers him a deal: assist her in retrieving a powerful stolen grimoire, 'The Book of Zero,' and she will grant him his human desire. Despite his disdain for witches, the mercenary embarks on a journey with Zero, a journey that intertwines the fates of a proud sorceress and a compassionate beast.