The Deptford Mice live a comfortable life in the skirting boards of an abandoned London house, with no humans or cats to disturb them. But something is lurking deep beneath the city. Something that threatens to destroy their cosy existence for good. In the dank sewers under the house lives a mysterious being, worshipped by a horde of bloodthirsty rats who cower in its presence... When a mouse called Albert Brown unwisely ventures down into the sewers one day, he uncovers a terrifying plot to awaken an ancient evil. Soon Albert's family and friends find themselves in a desperate struggle for their lives. Summoning all their courage, they must confront treacherous enemies and foul sorcery in a battle to save London and the world from eternal darkness.