Six years ago, the phenomenon known as "Goblin Night" gave 1 out of every 100 people the ability to transform into a monster, which came to be called a "goblin." As neither the government nor the police could oppose or keep up with them, some goblins committed evil acts with their power and caused society to crumble. In present-day Japan, Mamoru and Yuki, who possess the "goblin" ability, strive to protect their town - though the events that took place on Goblin Night, which resulted in the death of his brother, left Mamoru disinclined to trust or care about other people. And things only get worse when his brother's killer comes to town...