The Merlin Conspiracy by Diana Wynne Jones

Two young people from different backgrounds are thrown together when they uncover a secret plot among powerful magicians to manipulate the rules that bind parallel worlds; forced to flee, hide their identities, and learn to trust one another, they piece together layers of deception and political intrigue while discovering unexpected talents and allies. Racing through enchanted cities and bureaucratic strongholds, they must outwit both mundane and magical authorities to prevent a scheme that would remake lives across multiple worlds, with the story balancing sharp humor, clever twists, and themes of family, responsibility, and the misuse of power.

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