Casting attractive young film stars Elle Fanning and Douglas Booth, respectively, as Mary Wollstonecraft Shelley and Percy Bysshe Shelley makes Mary Shelley, director Haifaa al-Mansour's biopic of the mother of Gothic fiction, a kind of grandfather's paradox of the modern wave of eroticized young-adult romantic fantasy, reconfiguring the ancestor to match its descendant. The film's cleverest trick, foregrounded from the moment that a teenaged Mary meets Percy as a radical with scandalous notions of free love, is to suggest that, in YA terms, Percy himself is the monster with whom the bright, ambitious woman falls hopelessly in love. As such, Mary is only able to see his most intoxicating properties and none of his numerous dangers.
Despite the forthright opinions that Mary uses to goad her father, William Godwin (Stephen Dillane), and her conservative, demeaning stepmother, Mary Jane Clairmont (Joanne Froggatt), she quickly finds herself sidelined by the intensity of Percy's declarative politics and firebrand poetry. Percy's wooing of Mary hits the young woman so strongly that even when she discovers his abandoned wife (Ciara Charteris) and child, her feelings for him don't waver—though it doesn't hurt that Percy invokes his mother, who championed open relationships and freedom of personal choice in matters of the heart. Mary, so sarcastic and challenging around her family, finds in Percy a kindred spirit, failing to notice how she becomes a supplicant around him. That Mary continues to call him by his surname hints at an imbalance of power in their relationship, making her as much his partner as his number-one fan.
It abandons its fruitful investigation of belief systems in favor of a simplistic articulation of Mary's inspiration.
With Percy living increasingly beyond his means, Mary's self-awareness begins to reassert itself, and her mounting dissatisfactions plunge Mary Shelley's middle section into Gothic malaise. In the film's best stretch, Mary and Percy find themselves drawn into the torpor of Lord Byron (Tom Sturridge), who emits an air of musky indulgence, dressed in resplendent robes that nonetheless look as if they've never been washed. Stalking around his country chateau like a tomcat, Byron lays bare the emptiness of the free philosophy that Percy espouses, revealing the transparent narcissism of the well-fed aristocrat who needs a team of servants to uphold a lifestyle without work. Percy is enraptured, but Mary sees Byron for the crusty, stagnant hypocrite that he is, and in the process the last of her youthful idealism evaporates.
The film's final act, the one most directly concerning Mary's creation of Frankenstein, falls back into standard biopic rhythms. Al-Mansour and co-writer Emma Jensen abandon their fruitful investigation of belief systems in favor of a simplistic articulation of Mary's inspiration, complete with a montage set to a clip show of previously seen moments as she rattles off passages in voiceover. Mary Shelley ultimately feels perfunctory in its defenses of Mary's intelligence against doubting publishers who suspect her partner's guiding hand, reducing itself to the story of a single physical accomplishment and trading ambivalent character study for a tidy finale.