The Blue Castle is a novel by the author of Lucy M. Montgomery. The story follows the main character, Valancy Stirling, as she tries to put the pieces of her life together after finding out she only has about one year to live because she has a fatal heart condition. When Valancy, who is 29 years old and is an old maid by societal standards, finds out she is about to die, she starts living her life for the first time. She no longer cares what her mother or other family members say or think about her. Valancy vows to start living and experiencing everything she has always dreamed of having in life, including the man of the her dreams and a home of her own, which she refers to as the Blue Castle.

One of the first things that Valancy does upon arriving is to land a job as a caretaker of one of her former schoolmates who is dying of consumption. Valancy goes to live with the town drunk to care for his daughter, Cissey. While she is living there, Valancy also becomes close with one of Roaring Abel's friends, Barney Snaith.

After Cissy dies, Valancy asks Barney to marry her. She confesses that she's dying and Barney agrees to marry her. He takes her to live with him on his island up in the woods, which Valancy refers to as their Blue Castle. Valancy and Barney live over a year together and Valancy's heart problems seem to be decreasing until the point that the condition disappears.

After Barney saves Valancy from being run over by a train, they both realize that her diagnosis is incorrect. Barney's father comes to town without knowing that Barney is married. He tells Valancy of Barney's true identity. Valancy makes up with her father and Barney and Valancy end up living happily ever after.