At the age of sixteen, Mary Godwin, the very lovely daughter of philosopher William Godwin and outspoken feminist Mary Wollstonecraft, finds herself once more in the household of her much hated stepmother, writing for the London papers to help support the family. Into her life walks Percy Shelley, poet, aristocrat, and a great admirer of the works of both her parents.

Although Shelley is married, they declare their passion on the grave of Mary's mother, and elope to the Continent, bringing with them Mary's stepsister Clare. The year is 1814 and the escapade creates notorious scandal. The two lovers are the talk of England and Europe. The scandal only increases when they go to spend the summer with the poet Byron and his companion, Dr. Polidori. During that summer, bad weather brings recurring boredom, which even opium cannot relieve.

Byron challenges them all to a dare: whoever writes the most frightening story will be the winner. At first, Mary finds herself without any ideas, but one night she wakes to a vision of a monster by her bedside. The story of the monster and his creator, Dr. Frankenstein (who bears a striking resemblance to Percy Shelley) emerge from that vision, which ends the first act.

In the second act, Mary and Percy return to England with their baby, hounded by creditors. While they are there, Percy's wife Harriet commits suicide by throwing herself in the Thames, and leaves a letter for Shelley, pleading with him to give the children up to her sister Eliza. Percy and Mary finally marry in an attempt to gain custody of his children with Harriet. The attempt fails, and they return to Europe.

Though Frankenstein is a great success, Mary's flesh and blood children with Percy die one by one. The Monster, once a passing vision which retreated by daylight, becomes an increasing presence in her life. The Monster appears, with one of their dead children in his arms, just as he did in Mary's novel Frankenstein. Mary fears the Monster has killed the child. Percy, saddened by his own stalled career, always contrasting it with Byron's and Mary's success, reproaches Mary with coldness. With great trepidation, Mary announces a new pregnancy. The Monster becomes an increasing presence, looming in each scene, coming between them. Finally, their united fate overcomes them. The Monster and Shelley struggle on a boat in Lake Geneva; Shelley drowns.

After Shelley's death, Byron acts as agent for Percy's father, Lord Timothy. Lord Timothy wants to have his son's only living child sent to him for rearing, offering her an allowance if she accepts. When the penniless Mary refuses, he forbids her to write anything about her scandalous elopement and subsequent marriage to Shelley, threatening to revoke all support for herself and her child if she continues her planned autobiography. Mary insists that she will keep her child at any cost. After Byron leaves, the Monster demands to know why she will do anything for her child by Shelley, but continually rejects him. They struggle; Mary ends by finally accepting the Monster, her own creation.