A midwife is summoned to attend a childbed. The baby is born, and she is given an ointment to rub in its eyes. Accidentally, or through curiosity, she rubs one or both her own eyes with it. This enables her to see the actual house to which she has been summoned. Sometimes a simple cottage becomes a castle, but most often, a grand castle becomes a wretched cave.
In the variant Andrew Lang included, the woman saw a neighbor of hers, kept prisoner as a nurse, and was able to tell her husband how to rescue her, pulling her down from riding fairies as in Tam Lin.
Soon, the midwife sees a fairy and admits it. The fairy invariably blinds her in the eye that can see him, or both if she put the ointment in both eyes.