In modern Japan, a sudden emergence of dangerous demon beasts has caused widespread fear and disruption. These creatures target females, and their bodily fluids are lethal when injected into humans. This crisis prompted the development of a technology to absorb and harness the energy of demon beasts, but it's highly unstable. To optimize this method and avoid wasting resources, researchers developed the S-TRAN(C)XE System, also known as the Succubusystem, which can repurpose absorbed energy into an enhanced suit. During the system's first human trial, the research facility was attacked by demon beasts, leading to chaos and the death of a girl. However, she later awakens as an S-TRAN(C)XE, a hunter of demon beasts.