Mary Wollstonecraft dies shortly after giving birth to her daughter Mary Godwin [Elle Fanning]. Sixteen years later, Mary Godwin [Elle Fanning] is living with her father, William Godwin [Stephen Dillane], her stepmother, and her stepsister Claire Clairmont [Bel Powley].
A tense clash with her stepmother pushes the family to send Mary away, but William Relents and sends her to Scotland to stay with a good friend and his daughter, where Mary [Elle Fanning] meets and falls in love with the radical and unconventional poet Percy Bysshe Shelley [Douglas Booth], who is already married.
Called home under the pretense that Claire is on her deathbed, Mary returns to find Claire confessing it was a ruse to bring her back. While at home, William Godwin [Stephen Dillane] brings Percy Shelley [Douglas Booth] into the household as a new student, and the two rekindle their affections.
Mary and Claire are confronted in the street by Shelley's wife and daughter; the wife begs Mary to stay away from Percy. Mary lies, insisting that Shelley is only her father’s student, while Percy explains that his wife and child are “only that in name,” and that he supports them financially but has nothing to do with them, all the while claiming to love Mary.
The couple elopes, taking Claire [Bel Powley] with them. Shelley's lack of money forces them to borrow against his wealthy father’s estate to set themselves up in lavish fashion in Bloomsbury. At a dinner party, Shelley flirts with Claire, and Mary encounters a friend who later makes advances toward her. When Mary complains about the flirting, Shelley tells her that lovers should be free, and he demands the same freedom for himself. He even calls her a hypocrite, and Mary’s disappointment in him grows.
One night, Shelley's creditors arrive unexpectedly, and the trio must flee in the rain, finding cheap lodgings. Mary gives birth, but the baby does not survive long. Claire announces that she is pregnant by Byron, and that he has invited them to stay with him at a villa near Geneva. When they arrive, Byron makes it clear that the invitation is more wishful thinking than a real arrangement, yet he asks them to stay.
Bad weather keeps them indoors for days, and Byron challenges the group to write a ghost story; this sparks Mary’s imagination and she dreams of galvanism. A message arrives that Byron’s wife has just drowned herself. Throughout the visit, Byron treats Claire with increasing contempt. She confronts him, but he shrugs it off, quipping that their affair was “a lapse in judgement.” He promises to provide financially for Claire’s baby, but nothing more.
The trio returns to England, where Mary begins writing Frankenstein. Publishers refuse to accept the manuscript under a female author, deeming the subject matter unsuited for a lady, but a foreword by Shelley helps it pass as anonymous. The book becomes a success, initially credited to Shelley until he publicly reveals the true author. The couple’s bond is rekindled.
Mary’s father later ensures a second publication of Frankenstein under her own name, securing a solid income for Mary. In the final shots, Mary, dressed in black, walks with a young son, and an afterword explains that Mary and Shelley married and stayed together until his death at 29, with Mary never remarrying.
> "a lapse in judgement"
