Helen McCarthy writes: "A Japanese teenager visiting America witnesses an assassination and is kidnapped. The secret organisation Inferno erases his memory and brainwashes him through a combination of drugs and hypnotherapy, exploiting his natural survival instinct to make him into a deadly weapon. He is the second experimental assassin created by Scythe Master, so he is given the name codename Zwei.
Scythe Master's first subject, Ein, is Zwei's trainer and Inferno's top assassin – the Phantom. Outside her work, she is completely apathetic, but she and Zwei slowly form a relationship that awakens her forgotten past and sends her on the run. Zwei finds his own protégé, Drei, and continues his murderous rise until tragedy turns him rogue. Inferno sets out to reclaim its two lost sheep or slaughter them, setting the stage for a bloody resolution."