Mary Shelley was born at 20 minutes to midnight on 30 August 1797, at the top of a house in the Polygon, Somers Town, an aspirational address before the arrival of the railway, after which it became a notorious London slum. A healthy baby, no sooner had she arrived than her mother, Mary Wollstonecraft, sent a message to her husband, the radical writer William Godwin, at work in rooms nearby, to come and meet his new child. Wollstonecraft, the author of The Vindication of the Rights of Woman, did not hold with long confinements; the introductions having been made, she intended to rest only for a few more hours. The following evening, she planned to join him for dinner as usual.

Back at home, however, Godwin found himself waiting and waiting downstairs. All was not well, after all: the afterbirth had not come away and, shortly before two in the morning, the midwife, Mrs Blenkinsop, asked him to get help. An hour later, he returned with the man who would ultimately kill his wife, Louis Poignand, a licentiate in midwifery at the Royal College of Surgeons. Poignand removed the afterbirth bit by bit with his unsterile hands, thus giving his patient the infection from which she would die 11 days later. It was a horrible end: so much vomit and shit and blood, though the animal detail that most lingers in my mind is the fact that, the baby having been sent away to safety, puppies were brought in to drain off Wollstonecraft’s milk.

Is it possible to trace the “weird nativity” of Shelley’s celebrated novel Frankenstein, which she would begin writing only 18 years later, all the way back to this horror show? The poet Fiona Sampson believes that it is, seeing in her description of the creature’s birth – guttering candles, rain pattering at a window, a lifeless thing lying at a man’s feet – details that better suit a deathbed scene. But even if you think this a stretch (so many influences were upon Mary as she set about magicking, at the behest of Byron, what was supposed to be “a ghost story”, not least the fact that she was by then a mother herself), there is no doubting the impact of Wollstonecraft’s death on her life. It runs like a thread through everything she does, and everything she is, and while Sampson’s sense of this may sometimes be a touch 20th century – her recourse to what “the psychologists” have to say can be wearing – it is also the chief virtue of her daringly swift and enjoyably irreverent retelling of Shelley’s life. At the heart of her biography lies a paradox, which is that its engine is powered by absence and loss. Even when the action is at its most frantic, Sampson never loses sight of the gaping void below.

After Wollstonecraft’s death, the grieving Godwin unwittingly destroys her reputation with his (rather too honest) memoir of her unconventional life, after which, in 1801, he marries his neighbour, that mistress of reinvention, Mary Jane Clairmont. His family now swells to include not only his new wife, Mary, and her half-sister, Fanny Imlay, (Wollstonecraft’s daughter by her lover, Gilbert Imlay), but Clairmont’s illegitimate children, Charles and Claire, too; soon, moreover, there will be another baby, William. Mary and Claire, born a few months apart, are best friends. Mary and her new stepmother, however, are soon at odds. She will come, somewhat unfairly, to loathe her, seeing her as meddler and a barrier to her father.

It is a busy household and a precarious one (Godwin is forever in debt). No wonder, then, that at 16 the clever and precocious Mary is susceptible to the escape route that is love. She falls too quickly for Percy Shelley, a new associate of her father, her heart dashing towards him, her ears hearing only what they want to, and they elope, crossing the Channel by night, so eager are they to get away. But what’s this? Mary’s stepsister is, it seems, also in tow. Is she a chaperone? A confidante? Or is she just another pawn in Shelley’s game of Free Love? Mary Jane dashes to France hoping to save her reputation – she knows her daughter will be collateral damage once the elopement is made public – but it is no good. Claire decides to stick with Mary and Percy, like glue.

Sampson is hard on Claire, soon to be the lover of Byron, by whom she will have a daughter; I prefer Daisy Hay’s more sympathetic depiction in her 2010 book Young Romantics. It seems pitiless to describe her repeatedly as a “man’s woman”, given the way the world worked then. But perhaps there’s no time, here, for spare understanding. So much is happening. The trio cross Europe, only to have to return, broke, to England. Mary has a baby girl, who dies, and then a son, William (he will soon die, too, as will her daughter, Clara; only her fourth child, Percy, lives into adulthood). Fanny Imlay (more collateral damage) kills herself, as does Shelley’s wife, Harriet. Throughout it all, Mary is stoic, sealed off, her emotions seemingly cauterised. Her hopes lie with Shelley and Shelley alone, and her focus is on him, by which I mean, mostly, on his philandering, which she affects not to notice. Does she even care that when he marries her, it’s only a means to help him get custody of his children by Harriet?

It is against this backdrop that Frankenstein, begun in Byron’s villa on Lake Geneva, is completed, a book born, perhaps, of unconscious displacement. But while Sampson’s life is published to mark the novel’s 200th anniversary, she has relatively little to say about it now (what I like most about her book is her willingness to play about with time, a corollary of her reluctance ever to detour from exciting events). Mary walks away from her literary fame once her tale is out in the world, returning with Percy and Claire to Italy, and so does Sampson, temporarily. Her subject’s fatal attraction, after all, is not for reviews or the admiration of her peers. It’s for charisma and her husband still has plenty of this, his self-absorption and infidelity apart.

And so the whirl continues. When Percy drowns in 1822, his boat lost in a storm off the Ligurian coast, it comes almost as a relief: at last, you think, Mary will return to herself. But, no. Her grief is “hyperbolic”, for now he can be the man she always dreamed of: unimpeachable beyond the grave, his heart, recovered from the pyre on which his body was burned, kept in a silk purse about her desk. Her widowhood, rightly foreshortened on the page by Sampson, will be long and strange, inhabited until her death in 1851 by ghouls, rip-off merchants, a hateful father-in-law and an unimpressive son. What did she feel when she saw Richard Brinsley Peake’s production of Frankenstein on the stage in London in 1823? Sampson thinks she sounds girlish when she tells Leigh Hunt: “I found myself famous!” To me, though, she seems as cool as ever: a woman merely going through the motions. Her life was far more wretched than that of the monster she created and, for this reason, she was always able to keep it in perspective, dull yellow eyes and all.

• In Search of Mary Shelley: The Girl Who Wrote Frankenstein by Fiona Sampson is published by Profile (£18.99). To order a copy for £16.14 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99