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.