Switzerland, 1816. A volcanic eruption in Indonesia envelopes the whole of Europe in ash and cloud. Amid this “year without a summer,” 18-year-old Mary Shelley and her lover, Percy Bysshe Shelley, arrive at Lake Geneva to visit Lord Byron and his companion, John Polidori. One famous evening, Byron issues a challenge to write the best ghost story. Contemplating what to write, Mary recalls another summer, when she was 14. Scotland, 1812. A guest of the Baxter family, Mary arrives in Dundee, befriending young Isabella Baxter. As their bond deepens, Mary and Isabella’s feelings for each other intensify. But someone has been watching them --- the charismatic and vaguely sinister Mr. Booth, Isabella's older brother-in-law, who may not be as benevolent as he purports to be.