In London, early in the nineteenth century, five-year-old Mary Godwin, daughter of philosopher William Godwin, plays with her sister Fanny, mourns her deceased mother, and marvels as a hot air balloon lands not far from the Thames. Nearby, in Sussex, eleven-year-old Percy Shelley entertains his three sisters by telling them stories and performing tricks with chemicals and fire.

A few years later Mary and Percy meet and fall in love in the Godwin bookshop near Black Friar's Bridge. At first their romance seems doomed--Percy is a well-known atheist and already has a wife, and Mary is only seventeen and a under the care of her father and his overbearing second wife. But they consider such impediments trivial and are soon on their way to Ireland, to Switzerland, and across Europe (with Mary's flighty half-sister Claire in tow).

Upon reaching Lake Geneva they find lodgings near where the notorious poet Lord Byron and his peculiar personal physician John Polidori are staying--the same Lord Byron Claire seduced back in London, her reasoning being that if Mary can have a poet, why can't she? And so begins the summer when Mary Shelley will begin writing her novel about a reanimated corpse, Percy and Lord Byron will debate politics and poetry in the midst of lightning storms, Polidori will begin writing his novel about a man with a taste for human blood, and snow will fall in the middle of July.