The Deptford Mice live a cosy life in the skirting boards of an abandoned London house, with no humans or cats to disturb them. But something lurks in the dank sewers beneath the city – a mysterious, powerful being, worshipped by a horde of bloodthirsty rats who cower in its presence…
When a brave mouse ventures down into the sewers, he uncovers a terrifying plot to awaken an ancient evil.

Soon his family and friends must summon all their courage to confront treacherous enemies and foul sorcery in a battle to save London and the world from eternal darkness.