At twenty-nine, Valancy Stirling is an "old maid" trapped within the tedium and troubles of family life. She escapes by building the imaginary world of the Blue Castle.

When she is diagnosed with a serious heart ailment, Valancy realizes she has never been happy. She doesn't want to waste what time she has left. She seizes the moment. This catalyst for change leads her on towards relationships and entanglements far beyond what she could have predicted, including with a mysterious man known as Barney Snaith.

This is one of the few works by Lucy Maud Montgomery written for adults.