Half Bad is set in modern-day England, where witches live alongside humans without magic gifts known as fains, who are lagely unaware of the existense of witches. There are White witches, who tend to integrate with fain society and use their gifts for the sake of others; and Black witches, who usually distance themselves from fains and have a rather nasty habit of murdering each other. If a Black witch is caught by the White witch organisation known as the Hunters (a group whose sole purpose is the capture and eradication of Black witches), they will face Retribution, which is basically just a fancy word for magic torture.
Half Bad follows the early life up to his seventeenth birthday of a witch named Nathan Byrn, whose mother (Cora Byrn, deceased) was a White witch and whose father (Marcus Edge) was a Black witch - in fact one of the most notorious Black witches of all time, having killed many other witches, both White and Black, and stolen their gifts. Due to this, Nathan is persecuted by the council of White witches, and has more and more restrictions placed on him with every passing year. Eventually Nathan is separated from his family by the council, and is forced to live in a cage and train to kill his father - who once had a vision of a future in which Nathan killed him.
Nathan manages to escape but is then faced with the fact that he must have a Giving ceremony on his seventeenth birthday, otherwise he will never have any magical ability - and rumour has it that a Black witch who does not have a Giving will die. However, the ceremony requires the blood of either Nathan's parents or grandparents, and with his grandmother now dead he must either find his father or seek the Black witch Mercury, who stole the White witch blood stored by the council.