An original medical murder mystery surrounds an unknown killer coccus (bacteria), that leaves no trace in the victim's body, which enables it to be used as the perfect weapon to kill. Fujimaki Tatsuomi is a pathologist working at the Pathlogy department at Meikyo Medical University Hospital. He's looked down upon by the other staff around him, and his relationship with Maho his wife, a former nurse, has cooled down.

After a patient's suspicious death, he makes a new discovery; when the deadly bacteria "Gift" invades the body, the patient soon dies, but the "Gift" leaves no trace thereafter, enabling it to be used as the perfect weapon to kill. As a result of this discovery, Fujimaki's life dramatically changes. While searching for a way to treat his ailing wife's heart disease, he becomes embroiled in a conspiracy surrounding the killer bacteria "Gift."