The Original Frankenstein, By Mary Shelley with Percy Shelley ed Charles E Robinson

Mary Shelley's Frankenstein was famously inspired by telling ghost stories with Percy Bysshe Shelley and Lord Byron during a cold, wet summer in the Swiss Alps. It continues to serve as shorthand for the dangers of reckless scientific advance, yet literary historians have never been able to agree on its origins. Could Mary Shelley, an unpublished 18-year-old, really have written the novel? Or was Frankenstein's monster her future husband's creation?

The editor of The Original Frankenstein, Charles E Robinson, presents a convincing case for crediting the novel to "Mary (with Percy) Shelley", revealing the major changes PBS made to Mary's first draft. The manuscript is now in Oxford's Bodleian Library, having been secured for the nation as part of a £3.85m purchase of Shelley family papers in 2004, and Robinson has used it to distinguish thousands of words that PBS changed, added or deleted.

The resulting book presents two versions of the novel, one as Mary Shelley first drafted it and the other with PBS's revisions in italics. His changes amount to almost 5,000 words of a 70,000-word novel and dramatically alter the story. PBS deleted Mary's religious references, developed the novel's scientific and political themes, changed the motive behind Victor Frankenstein's voyage to England and added such telling details as the description of the monster's "lustrous black" hair.

Mary seems to have accepted his suggestions, which Robinson argues were made while she was still writing the first draft, perhaps as the couple worked on the manuscript together in bed. This allowed her to respond to his revisions as she continued writing and PBS made fewer alterations to the later stages of the novel. The Original Frankenstein highlights the changes and presents two versions of the draft, both pre-dating the 1818 first edition. The version that was eventually published incorporated changes made at a later stage of revision, including a different division of chapters and volumes.

At many points in the story PBS's involvement suggests the work of a helpful and perceptive editor, tightening the plot, developing its main themes and ultimately producing a better novel. Back in England, PBS was also the one who found a publisher and wrote the preface, while Mary was heavily pregnant with their daughter Clara.

However, his more significant contributions run deeper. He made the monster far more human, opening the novel up to Freudian interpretation. PBS's monster feels compassion, "wonder" and "emotions" he is "unable to bear", speaks of "the agony of my grief" and describes a gunshot wound which "shattered the flesh and bone". In Mary's draft, Victor Frankenstein imagines "the fangs of the monster already grasping my neck", but PBS substitutes "fingers" for "fangs". The monster tells his creator that he has "learned to work mischief" from the way he has been treated: in PBS's version, it is society that creates the monster.

For all its lasting appeal, Frankenstein never was, and never will be, a perfect novel. The Original Frankenstein shows that PBS's best attempts could not repair the many incoherent parts of the narrative. Suspension of disbelief is easier at the creation of the monster than the awkward twists of the plot that follow, including the monster's fortuitous discovery of Frankenstein's papers in his pocket and the scientist's need to travel to England to create a female monster. Modern readers are unlikely to thank PBS for trying to polish Mary's more colloquial style, which only makes the prose more ponderous. Meanwhile, the minority of scholars who remain convinced that PBS dictated the entire novel to his future wife will probably refuse to be persuaded by the overwhelming evidence otherwise. The title of John Lauritsen's 2007 book, The Man Who Wrote Frankenstein, says it all.

This is unfortunate, because the theories that deny Mary Shelley's authorship are much less interesting than the true story behind Frankenstein. If the novel shows the dangers of solipsistic quests for knowledge, its creation represents a kind of joint authorship. Inspired by Byron's ghost-story competition, it draws heavily on the novels of Mary's father William Godwin and blends many elements of the Shelleys' biographies. Frankenstein's beloved cousin, Elizabeth, is named after Shelley's favourite sister and the monster's first victim shares a name with the Shelleys' first son. The novel is set in places the Shelleys had visited together and PBS's frequent fear that he was being followed contributes to the monster and his creator's obsessive pursuit of each other.

Mary later described the novel as her "hideous progeny" and its preoccupation with tragic births reflects the fact that her mother, Mary Wollstonecraft, died from complications a few days after giving birth to her. The novel the Shelleys wrote together represents a remarkable act of literary homage and collaboration and Charles E Robinson's revealing new edition allows modern readers to be there at its creation.