Havva, age 7, and her brother Peter, age 9, have recently moved to the town of Havre de Grace on the upper Chesapeake Bay. On the day they arrive, they ride their bikes to explore their new community and discover an old run-down boathouse. After entering, Havva opens a dusty trunk to find an old map, while Peter leaves to explore outside. To her surprise, Havva encounters Gracie, the sea monster who lives on the map. She and Gracie start a conversation, and when Peter returns, he learns that he can communicate with her too. Gracie thrills the children by taking them with her as she relives her stories of fierce battles in Chesapeake Bay history, and moving encounters with her marine friends who are struggling to survive in polluted water. The children learn from Gracie and in the end, learn about themselves and the world around them, as well as the power of the imagination.