There's an irresistible underdog appeal to the theme of a little girl becoming a tooled-up warrior assassin, and Tim Clare's 13-year-old heroine Delphine Venner joins a noble roll-call which includes Matilda from Leon, Arya Stark from Game Of Thrones and Hit-Girl from Kick-Ass.

It's England in 1935. Daddy has had a breakdown, and the Venner family (mum, dad and Delphine) take up residence at Alderberen Hall, which plays host to the Society for the Perpetual Improvement of Man, a quasi-mystical cult led by Ivan Propp, a dance teacher of indeterminate but exotic background, who leads his small group of adherents through yoga-like exercises. As rude and arrogant as these people are, it is hoped that, with Propp's help, Mr Venner will recover his mental stability.

There's not much to do for a headstrong young girl with a head full of war stories but pester her tutor for books about poisons, prowl the grounds and explore the house, in which she finds a network of secret passages and spyholes which enable her to eavesdrop on the men at the heart of this community. Delphine discovers that they are involved in some skulduggery aimed at taking Britain into a war. Without any hard evidence, and with the estate's gamekeeper as her only ally, she must somehow expose these subversives and bring them to justice.

Ah, if only it were that simple. An overheard remark made by one of these men to another - "We do not speak of honour, dear brother. We speak only ... of will." - is an early hint that John Buchan is less of an influence here than Aleister Crowley, and Tim Clare pulls off an audacious genre-switch at the mid-way point, whereupon hordes of bat-like creatures from another dimension descend on Alderberen Hall and turn it into a war zone. There follows a full 200 pages of non-stop action, tension and peril of such intensity that one can easily imagine Clare hyperventilating at his keyboard.

Considering that the one swear word in the book is tastefully blanked out, it can be a surprisingly visceral novel (some human-on-human violence involving Delphine being arguably the most unpleasant), a tour-de-force of breathless thrills in which a supremely plucky heroine stands up to the forces of darkness armed only with a hunting rifle and some grenades made from jam tins, and it's one which dares you to read it in one long sitting.