When Berrin is thrown out of his dormer to be eaten by the wild beast of the night, he is, instead, rescued by an grimy-skinned boy who takes him down into the storm water tunnels beneath the city. Here he joins the Rats, a gallant group of children who are fighting back against Malig Tumora, the ominous force who has taken over the city and enslaved all the grown-ups. Traveling at breakneck speed along the underground pipes, the children occasionally venture onto the surface to sabotage and seek ways they can defeat their enemy, guided by the wisdom of Ferdinand, an older boy who is trapped in the darkness by his size.

Berrin and his new friend Olanda become two of their best fighters until they are captured by the ruthless Gadger Red. In making their escape, they steal a map that will help them in their struggles, but id the map worthy giving up their greatest secret?