In the final book of the series, the Rats take on their arch enemy, Malig Tumora, in a struggle to the death

Berrin and the Rats are attempting to breed large numbers of the moth rescued from the lair of the mountain beast, in the hope that the resultant caterpillars will devour the flowers that give Malig tumora control over the adults. Meanwhile, Malig tumora is mounting a renewed offensive. New robotic dangers appear: vicious new creatures threaten the Rats in their underground tunnels. time is also running out for Berrin's uncle, the fabled Ferdinand, who is still trapped underground. If Berrin does not free him from the tunnels soon, he will die. A do-or-die showdown ensues between the Rats and Malig tumora's fearful henchmen, the Gadges. But as the struggle finally swings the way of the Rats, Berrin faces his biggest dilemma. His parents live and work in the ominous Obsidian Building, which also houses Malig tumora. to destroy his enemy completely, Berrin must risk causing the death of his own parents ...