MARY SHELLEY tells the story of Mary Wollstonecraft Godwin (Elle Fanning) - author of one of the world's most famous Gothic novels 'Frankenstein' - and her fiery, tempestuous relationship with renowned romantic poet Percy Bysshe Shelley (Douglas Booth.) The pair are two outsiders constrained by polite society but bound together by a natural chemistry and progressive ideas that are beyond the boundaries of their age and time. Mary and Percy declare their love for each other and much to her family's horror they run away together, joined by Mary's half-sister Claire (Bel Powley.) In the midst of growing tension within their relationship during their stay at Lord Byron's (Tom Sturridge) house at Lake Geneva, the idea of Frankenstein is conceived when a challenge is put to all houseguests to write a ghost story. An incredible character is created, which will loom large in popular culture for centuries to come, but society at the time puts little value in female authors. At the tender age of 18, Mary is forced to challenge these preconceptions, to protect her work and to forge her own identity..