The Castle in the Sea is on an island off the coast of California. The time is the present. The heroine, Lucinda de Cabrillo y Benivides, has just become one of the richest young women in the world due to the horrible death of her father. However, she must deal with the legacy of madness her father left behind before she can begin to deal with her great wealth…. Lucinda's "novio" arrives, a young man whom she has never met yet to whom she has been promised since her childhood. Next a doctor is engaged to help Lucinda with her melancholia, then a woman is employed to be her constant companion. And last of all, her ever present guardian, Ricardo Villaverde, who seems even more mad then her dead father, watches over her. Lucinda must decide who among them is her friend and who seeks to harm her...