Mary Shelley was but 18 when she began work on a stunningly original, complex, literary work about a scientist named Victor Frankenstein and the unnamed creature he pieced together.

And this was in 1816! Surely there’s rich cinematic material to be mined from the tale of a teenager creating a legendary fable some 200 years ago.

But there’s so much more.

When Mary was 15, the poet Percy Shelley (who was married and had children) started an affair with her. They ran away, taking Mary’s stepsister Claire Clairmont with them.

Claire took up with Lord Byron. THE Lord Byron.

The relationship between Percy and Mary was unconventional and tumultuous, to say the least. And Mary’s family history was complicated — to say the least.

And yet for all those scandalous, hedonistic historical ingredients, “Mary Shelley” is a dull, tame and disappointingly conventional biopic that almost never misses an opportunity to miss an opportunity.

Here we have a badass, emotional roller coaster of a rock ‘n’ roll story from the early 19th century, with myriad parallels to modern-day issues — and it gets the understated, prim-and-proper, period-piece PG-13 treatment.

Director Haifaa Al-Mansour opts for a static, safe, borderline corny approach throughout, from the meticulously appointed and lavish (but admittedly gorgeous) set designs to the breathless voice-overs to the overly stagey moments when characters take turns hurling ultimatums and declarations of intentions at one another.

Elle Fanning is a talented and versatile actress who’s miscast as Mary. Fanning delivers competent work, but her performance comes across as calculated, and is often sorely lacking in passion and fire.

We’re introduced to Fanning’s Mary Godwin as a smart, mildly rebellious teenager, living under the stern rule of her renowned author-philosopher father William (Stephen Dillane), and forever at odds with her horrible stepmother, Mary Jane (Joanne Froggatt). Mary hates her stepmother, resents her father and says she “killed” her mother, the revolutionary feminist author Mary Wollstonecraft, who died 11 days after giving birth to Mary.

So yes. Mary has issues.

Enter Percy Shelley (Douglas Booth), a handsome, preening, narcissistic wordsmith and peacock who effortlessly seduces Mary and even charms his way into her father’s good graces — at least initially.

The 15-year-old Mary falls in love with 21-year-old Percy, who has conveniently omitted the details about his wife and children during their “courtship.”

(Mary learns about Percy’s deception when his wife confronts her on the street, introduces Mary to her young daughter and says, “Evidently you are a stranger to scandal, Miss Godwin. Did you know I ran away with Percy when I was a girl?” Mary’s reaction is cold and uncaring.)

Percy is also in love — with himself, and with his grand ideas about open relationships and free love. (When Mary tells Percy his friend tried to force himself on her, and she tells him she fought off the jerk, he expresses his disappointment in her “conventional” reaction to the attack and berates her for not being open to the creep’s unwanted advances.)

With their thick and lustrous hairdos and their dramatic makeup and their carefully coordinated and flamboyant wardrobes, Douglas Booth’s Percy and Tom Sturridge’s Byron strut and preen about like the Mick Jagger and Keith Richards of the early 19th century. (I kept hoping Mary would say, “And another thing: I’m wearing this drab outfit and I have this mousy hairdo, while you two idiots have the fancy haircuts and the rouge and the guy-liner and the tailored suits. What’s up with THAT!”)

“Mary Shelley” picks up a little dramatic steam when Mary is inspired by certain life experiences to write “Frankenstein: or, the Modern Prometheus,” only to be met with skepticism from publishers who doubted she was the true author of the material. The first edition was published anonymously — but the preface was written by Percy, further reinforcing the belief Percy was the real creative force behind the novel.

Even when Mary finally gets her due, the film badly fumbles the moment.

To paraphrase Frankenstein’s monster: AAARRGGH.

★★

IFC Films presents a film directed by Haifaa Al Mansour and written by Al Mansour and Emma Jensen. Rated PG-13 (for sexuality and thematic elements including substance abuse). Running time: 121 minutes. Opens Friday at the Gene Siskel Film Center.