Lisa Duncan, a seventeen-year-old high school senior, has an assignment to tour a historic site and write a report about it. She chooses Mirabelle Manor, a large estate overlooking the ocean, built by a nineteenth-century sea captain for his wife. During the tour, she thinks Mirabelle's portrait is watching her. One of the items on display is a diary, open to a passage about going to a place called the unicorn tree to watch for ships. This appeals to Lisa because her brother is currently on a commemorative voyage, sailing across the Atlantic. When news arrives that his ship is lost at sea, Lisa's interest in the diary changes from curiosity to determination in hopes of discovering what happened to her brother.