The book has been translated into a number of languages, including Czech, Estonian, Italian, Russian, Spanish, German (as "Das geheimnisvolle Paket"), and Ukrainian.

While taking a tour in an inflatable dinghy on their first day, the children rest on a sandbank, where Penelope encounters a strange, paper-wrapped parcel from which two voices emanate.

Due to space restrictions, the beast population must be strictly regulated; but recently the resident Cockatrices have risen in rebellion and, with the assistance of their Toad egg-breeders, are working on drastically expanding their numbers and taking over Mythologia.

Upon questioning, the Toad, who introduces himself as Ethelred, divulges the existence of a drain which leads into Cockatrice Castle, and is made to switch his allegiance by having him titled a "master counter-spy".

Its use is made all the more vital because the Weasels are natural cowards and the herb would make them more belligerent, but the only known supply in Mythologia is to be found in the Mandrake Forest, on Werewolf Island in the Singing Sea.

After escaping the castle and sharing their discovery with H.H., Tabitha and Dulcibelle, the children, Parrot and Ethelred set out to gather allies: first in Weaseldom, the Weasels' domain under their lords Duke Wensleydale and Duchess Winifred; then with the Griffons, Mythologia's gold miners.

While Parrot, Peter, Simon and most of their allies assault the castle directly, a small strike force led by Penelope and Ethelred infiltrates the dungeons through its drainage system and secures the Books of Government before the Cockatrices can destroy them out of spite.