Set in 1823, when Mary Shelley's Frankenstein had become a bestseller, this novel imagines the life of Mary's real-life childhood friend Isabella Baxter Booth, who lived in Broughty Ferry.

In a letter to Leigh Hunt describing an encounter with Isabella in later life, Mary wrote about her as being "disturbed in her reason" and Lesley McDowell makes this the premise for an exploration of madness and the prevailing attitudes to it in the 19th century, with a particular emphasis on the treatment of women deemed to be "of unsound mind". It is a rich historical context. On the one hand there is the association, deeply ingrained in the Victorian mind, of madness with immorality and possession by the devil, and on the other we have the beginnings of a more enlightened approach to insanity as a treatable illness and the birth of modern psychiatry.

"We are a strange mix here, Mary. We are well-educated in parts, we believe in the progress of man, in the cultivation of his mind and his ability to reason. But some still believe in sentiment and sorcery … It is the old and the new, warring with each other, but the new will win out. The men of enlightenment chase away old wives' tales."

This forward-thinking view is voiced by Isabella's husband, a scholar who becomes increasingly anti-social the more he steeps himself in his work, until he loses touch with the real world completely. He suffers from a chronic form of epilepsy, but he also comes to have severe mood swings and periods of delusional behaviour which climax in bouts of violence towards her. After unsuccessfully seeking help from her friend, Mary, Isabella leaves London with her young children and returns to her childhood home in Broughty Ferry. Her husband agrees to this, provided that she delivers a sealed letter to the Superintendent of the Lunatic Asylum in Montrose. She does so, convinced that this is a good sign and that her husband is seeking professional help for his illness. Unbeknown to her, the letter begs the doctor to treat Isabella.

So she comes to meet Alexander Balfour, a newly appointed, highly ambitious young doctor who seeks to make his mark and become immortalised as an innovative genius in the field. In Isabella he finds the ideal case study. Duping her into believing that she is his guest in the hospital and not a patient, he pretends that he is interviewing her in order to understand better her husband's condition.

During the many "sessions" which follow, he believes that he has a eureka moment, and begins to develop his radical theory of madness as being something contagious, where "the madness of a male spouse infects and causes madness in the female": "Whytt had developed that 'sympathy' of the nerves, acting upon each other, linked to each other, creating a system of consent between various parts of the body, a system that Cullen had then expanded.

"But the part of that theory that Cullen had smothered, he, Alexander Balfour, would expand and claim for his own! A sympathy of the nerves, taken further to a notion of infection. The infection of madness! He would be the successor to Whytt and Cullen, after all."

As this quote suggests, Unfashioned Creatures - the title is a quote from Frankenstein: "We are unfashioned creatures, half made up … [with] weak and faulty natures" - is well-researched, but never becomes a history lesson, and Alexander Balfour is a splendidly realised character.

In McDowell's capable hands, he becomes a delusional monster of his own making who is ruthless in his self-interest and symbolic of patriarchal Victorian society. In Isabella, McDowell creates an unreliable narrator with whom we sympathise deeply but also at times distrust. This maintains an engaging edge of ambiguity throughout.

Apart from being a highly intelligent historical exploration of madness and our attitudes to it, this novel is also a compelling account of the developing relationship between a psychiatrist and a patient, in which the former comes to exert an extraordinary power over the latter, with disturbing consequences.

McDowell's prose is dense but sharp, charged with urgency by her deep interest in her subject. As in Alexander Balfour's theory of madness, her passion for it in this novel is infectious.