Once again, their help is required to deal with a crisis: Tago the Ef-frock has announced that he is tired of his powers being constantly exploited. He has now gone rogue, and is threatening to unleash his magic to destroy all who oppose him if he is not given world dominance.
The wise old man, Wuchiwark, believes that the Underworlders’ only hope is an ancient parchment rumoured to contain a formula which will help them find a balance to the Effrock’s hitherto unchallenged powers. This parchment is stored in the Library of No Return, a mysterious place from which no one who entered has ever emerged. The children agree to undertake this quest in secret, as their old nemesis the CATTSS (the ruling council of the Underworld) has forbidden anyone to enter the library, and will go all out to stop the children from obtaining this sacred relic.
But as the children embark on their new adventure, they soon realise that all is not as it seems. And they begin to wonder if there is more to the parchment than Wuchiwark has revealed to them, and, more crucially, if their enemy this time is not the CATTSS, but really Wuchiwark himself...