Mystery of the Sea is an adventure novel set on the coast of north-east Scotland. Its narrator, a young man named Archibald Hunter, meets a woman with unusual gifts of second sight and becomes involved in a search for clues about a lost treasure linked to the days of the Spanish Armada.
The story combines romance, codes and ciphers, coastal scenery and a good deal of local atmosphere. Along the way come questions about fate, hidden knowledge and international rivalry at the turn of the twentieth century. It is a lively, somewhat rambling tale that shows Stoker's range beyond Gothic horror and his love of the sea and its legends.