In a parallel universe, Roddy (a.k.a Arianrhod), daughter of two magicians who serve the King of Blest, has traveled with "the King's Progress" her entire life. The King's Progress is a mobile Court that continuously roams the Islands of Blest (our England) to contain and control the natural magic in the world. Roddy and her best friend, Grundo, uncover a sinister plot involving Grundo's mother and the new "Merlin" – the magical governor of Blest – to take over the throne and the magic of the universe. When Roddy and Grundo try to warn the adults around them of the plot, they are not believed, and Roddy ends up making a spell to ask help of someone from another world – unfortunately, the only person she manages to find is Nick.
Nick Mallory (a.k.a. Nichothodes Koryfoides) is a boy living in our own England who dreams about becoming a Magid and travelling to other worlds. A Magid is a sort of magical policeman who travels between worlds and helps people. Nick finds himself accidentally wandering the dark paths between the worlds, where he finds Roddy and then the powerful magician Romanov. Nick finally makes his way to Blest when he finds Maxwell Hyde, Roddy's grandfather, who is a Magid. But Grundo's mother and the fake Merlin have been kidnapping all the most powerful witches and wizards in Blest – including Maxwell Hyde and both of Roddy's parents – and it is up to Nick, Roddy and Grundo to raise the land and stop the plot.
The main characters, Arianrhod Hyde (called Roddy) and Nichothodes Euthandor Timosus Benigedy Koryfoides (a.k.a. Nick Mallory) come from alternate worlds (The World of Blest and Earth respectively) and the book is written from their point of view (each one writing alternate chapters). The Merlin Conspiracy is set in the same multiverse as Jones' Deep Secret, although the only character in common with that book is Nick and his father, Ted Mallory.
Ilsabil and Isadora are two twin sisters whose mother and grandmother are hereditary witches. Maxwell Hyde is a Magid who travels between worlds. He lives in Blest London with Dora (Maxwell's daughter) and Toby (Dora's son).
The New York Times gave cautious praise to this novel, saying it "blends some New Age spirituality with the grand old Stonehenge/White Horse/Arthurian/Britain-as-Magicland material into a strong concoction", but felt that the characters could be more vivid and the mythology more inventive.
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