One April day, a middle-aged writer, who was raised a Protestant but is not religious, is surprised to find a stranger, wearing a plain raincoat, in her home. The intruder says that she is Mary, mother of God, and something in her direct simplicity announces the truth of her claim. May will be the busiest of Mary's year, and she is tired; she asks to stay for a week to rest. Over lunch of soup and sandwiches, the two women establish an arrangement that Mary will stay and that if her host chooses to write about the visit, she will call it a work of fiction.

As Mary quietly goes about a banal existence--shopping, napping, doing laundry, using the toilet--the writer embarks on an investigation of Marian history--contrasting the relatively few biblical references to the mother of Jesus with her enormous fame and heavy responsibilities over two thousand years. Why has Mary been venerated? How can she cope with the thousands of prayers sent to her each day? Can transcendence reside in the mundane tasks of female life?

By the time her guest leaves, the writer is not converted but she comes to the conclusion that faith includes the acceptance of uncertainty, and that writing is an act of faith.