Gurumin: A Monstrous Adventure stars a strong-willed girl named Parin, who is sent to live with her grandfather in a small mining town while her parents are off excavating an ancient ruin. With no other children living in this town, Parin is surprised one day to encounter a young girl being tormented by a stray dog. After Parin drives off the mongrel, she is shocked to discover the young girl is actually a monster, and like all monsters, invisible to adults. Parin is soon invited to the mysterious Monster Village, which happens to be hidden right behind her town. Parin has just started making monster friends when evil spirits, known as Phantoms, invade the village and monsternap most of the inhabitants. Parin soon learns of the Legendary Drill, a weapon hidden underneath the village that is powerful enough to destroy the Phantoms. Thus begins Parin's quest to reclaim the Legendary Drill and set the world right.