Peggy, Mike and Laura are staying with their horrible Aunt and Uncle while their eccentric father, Thaddeus sails around the world, accompanied by photo journalist Charlotte Clancy. But in the aftermath of an unexpected storm, Thaddeus and Charlotte are presumed drowned and the children's Uncle and Aunt see this as not only the perfect chance for inheriting Thaddeus's estate - but selling it.
After retreating to an island, the children befriend Jack, an orphan who has run away from home. Unbeknownst to the children, their father Thaddeus and Charlotte are still shipwrecked on an island themselves. Uncle Henry and Aunt Harriet having discovered that the children have run away, decide not to tell anyone in case it reveals their plans to sell the estate.
As the children take shelter in a secret hideout, they soon discover the identity of the ghost on the island to be a kind, old monk who has taken refuge. Aunt Harriet and Uncle Henry try somehow to prevent the authorities from closing in, having been alerted to their insidious plan by Lord Foggo, chairmen of the explorers club and mentor of Thaddeus.
Aunt Harriet and Uncle Henry continue to try and stop the authorities from finding the children. Meanwhile, Thaddeus and Charlotte find themselves facing unexpected dangers as they attempt to survive on the desert island and agree to place their differences to one side so that they can work together.
Thaddeus and Charlotte embark on a book tour, leaving the children with their wacky nanny - Ruby. While exploring, 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, who is being held by the King's brother who plans to usurp the throne.
While the children attempt to rescue the young prince, Charlotte discovers that Thaddeus has been kidnapped from their hotel on the book tour, and she sets out to solve the mystery.
The children continue to try and outwit the kidnappers and deliver the Prince home. Meanwhile, Charlotte continues to investigate the disappearance of Thaddeus.
Charlotte discovers that Thaddeus has been captured by the King of Baronia who believes that he was involved with the kidnapping of his son.
Charlotte accompanies Thaddeus on a lecture tour - where Thaddeus is revealing exciting details of the discovery of a rare species of spider. But the event is unexpectedly interrupted by sinister men who steal the spider. The children have gone on holiday to the Killimooin mountains in the heart of Baronia. But unknown to them, there are secrets within the pine forests.
After Charlotte is kidnapped, Thaddeus is bitten by a spider while trying to rescue her. Meanwhile, back at Killimooin Lodge, Laura is captured by some guards. The other children try to rescue her but are caught on camera and are trapped by one of the manic Clovis Monk's traps.
The children plan to escape and rescue Laura but soon discover that Barney, the Prince's bodyguard, is working for Clovis Monk. To her horror, Charlotte uncovers Monk's evil plan to make a world of living dead, and she is the first victim.
After Charlotte is captured by Block and held prisoner in the center of the mountain range, the children discover the evil Clovis Monk's plans. Thaddeus, who has set off in search of Charlotte, is reunited with the children - but can they solve the mystery and save the day - and Charlotte - in time?
Thaddeus reveals that his application to adopt Jack has been a success. The family take a break in an Irish castle run by the eccentric, volatile Ma Moon and her mysterious son, Fin Bar to celebrate. But when Charlotte proposes to Thaddeus, he disappears. But did he run away? And do the instincts of the children that the Castle is haunted have any firm foundations?
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. The children encounter a ghostly figure on horseback and are convinced more than ever that the Castle is indeed haunted.
Charlotte finds Thaddeus - much to his annoyance. Jack and Laura stumble upon a secret passage which leads to Ma Moon and Fin Bar's secret laboratory. And all are astounded by what the displaced Mother and Son are aspiring to achieve.
After realizing that they are part of a dangerous experiment, Thaddeus, Charlotte and the children try to foil the bizarre plans of Ma Moon and Fin Bar.
As part of the honeymoon celebrations, the children accompany Thaddeus and Charlotte to Africa, where they attempt to visit an ancient temple. But they quickly discover that no matter how hard they try, the group are constantly pulled towards the mysterious mountain which looms ominously and seems to draw them closer and closer. But towards what?
After a surprise attack by the inhabitants of the mountain during the night, Thaddeus and Charlotte are taken prisoner. The children begin to question if there is any validity to the native guides' belief that the evil spirits of the mountain are responsible.
The children are captured and held prisoner by a mysterious tribe of Silver skulls within the mountain. All concerned are devastated when Laura takes the place of the child goddess to preside over the apparent sacrifice of Thaddeus.
The family continues their attempts to escape the sinister and obscure Silver skulls in the mountain. And Ruby must conquer her fear of flying in order to save her friends. But can she achieve her risky plan in time?
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.