Number is not your average serial killer. Equipped with the ability to identify future murderers, Number takes the law into his own hands. He believes in ridding the world of destined killers. Simple, right? One day, he runs into Jihyeon, whose number changes constantly as she develops a special power of her own. Will the two work together to eliminate would-be murderers? Or will Jihyeon's very existence threaten Number's moral compass?