One day middle school student Sumire Takahana meets the mysterious Lucia Nahashi, who is part of a group called Venus Vanguard. From that day on, Sumire is drawn into Lucia's ongoing battles with "The Virus", an evil spirit that snatches people's souls away. Sumire discovers that she not only has the power to see the Virus, but that she can turn into a mindless living antibody, called "Berserker mode"--an ability that Lucia finds highly useful for her purposes. It's needless to say that these events turn Sumire's life totally upside down.