Peggy, Mike and Laura are staying with their Aunt and Uncle - Harriet and Henry - while their eccentric father, Thaddeus (accompanied by photo journalist Charlotte Clancy) sails around the world. In the aftermath of an unexpected storm Thaddeus and Charlotte are presumed drowned. Uncle Henry and Aunt Harriet see this as a perfect opportunity for inheriting Thaddeus' estate. Frustrated by their devious plans, Henry and Harriet take it out on the children in their care - causing them to run away from their wicked Aunt and Uncle.

Romance is in the air - much to the children's concern - when Thaddeus and Charlotte go off on a book promotional tour together, leaving Peggy, Mike, Laura and Jack in the capable hands of their wacky new nanny - Ruby. The children discover that nearby to their house a boy is being held prisoner and his identity is eventually revealed to be the kidnapped Prince of Baronia. The Prince has been kidnapped by the King's brother who plans to hold the boy ransom and usurp the throne himself.

Charlotte accompanies Thaddeus on a lecture tour - where Thaddeus is revealing exciting details of a recent natural world discovery which has been made into a rare species - the Malucu Devil Spider. The entire event is suddenly disrupted by the unexpected intrusion of sinister men - eventually revealed to be led by the enigmatic Block, who for some unknown reason has stolen the spider and transported it to a mysterious destination. Meanwhile, staying in the Killimooin mountains on holiday - in the heart of Baronia - is a wonderful experience for the children.

Thaddeus reveals that his application to adopt Jack has been successful. The family take a short break in an Irish castle run by the eccentric Ma Moon and her mysterious son Fin Bar to celebrate. Charlotte proposes to Thaddeus - but the next day he has disappeared... Charlotte goes off in search of Thaddeus. Ruby finds herself a reluctant participant in Ma Moon and Fin Bar's quest; the search for eternal youth!

Paul invites his friends to Baronia once again, and they are camped near a secret airfield where new Baronian aircraft are being tested with the help of Captain Arnold; but there seem to be unknown enemies who are seeking to sabotage this work, and this seems to be connected with a remote, inaccessible valley that the children stumble into almost by accident.