In response to a decrease in population, human corpses have become a major resource for society's labor pool. Necromancers are needed to animate corpses for various jobs. These corpses work reliably unless they encounter human love, at which point they go berserk and attack. Zen is the necromancer of his city, and he spends his time animating corpses and dealing with the messy aftermath of those cases of 'necrophilia.'
(Source: Kissmanga)