In a prologue to the novel, the narrator recalls legends of the time, 300 years ago, when the minister of the Kirk of Woodilee in the Scottish lowlands was spirited away by the fairies or – as some said – by the devil. The story opens in 1644 with the coming of David Sempill, newly-ordained minister of the Church of Scotland, to Woodilee, a parish passionate in its support of the Covenant. Sempill is less committed to strict doctrinal practices than many of the Covenanters, and he finds himself attracted to the creed of Mark Kerr, a fugitive and follower of Lord Montrose, supporter of the King and enemy of the Kirk. Soon afterwards, when Kerr is injured, the minister secretes him within the manse. One night in the feared Black Wood of Melanudrigill the minister stumbles across a diabolic rite in which figures wearing animal headpieces dance around a pagan altar. He later attempts to identify the ringleader and although unsuccessful manages to splash his clothes with pungent aniseed oil. The next day the wife of a prominent elder of the Kirk, Ephraim Caird, is discovered burning clothes on a fire which smells strongly of aniseed. The plague comes to Woodilee. Sempill labours to prevent its spread helped by a newcomer named Mark Riddel who, unknown to the locals, is in fact the fugitive Mark Kerr. Nursing care is surreptitiously provided by a shadowy figure whom the locals take to be a fairy but who is in fact Katrine Yester, niece of the locallaird, to whom Sempill is secretly engaged. Katrine contracts the plague and dies. A local woman is accused by a pricker of being a witch and in spite of the best efforts of Sempill and Riddel is tortured and killed. Sempill presents his evidence of Ephraim Caird's heresy to the presbytery, the Kirk's religious court, which rejects it as circumstantial and unreliable. In retaliation, Caird brings counter-charges against the minister for harbouring a fugitive, for associating with Mark Riddel, and for keeping the company of an unknown woman. Sempill is found guilty and is excommunicated and ejected from his ministry. On his way back from the hearing, Sempill meets Ephraim Caird near the Black Wood. He forces Caird to enter, kneel before the pagan altar, and to make his choice between Christ and the devil. The effort is too much for Caird who runs off in demented terror and is killed in a fall. The minister is never after that day seen again, giving rise to the legends mentioned in the novel's prologue. In an epilogue, it is revealed that Sempill and Kerr had ridden to Leith and had boarded the first available ship out of Scotland.
Early critics were quick to recognise the significance of the novel, which has since come to be regarded as Buchan's masterpiece. The Spectator called it "this powerful, charming and spiritually earnest novel which almost enables Mr Buchan to be called a modern and terse Walter Scott", and the Glasgow Herald thought that it "must be adjudged the greatest of Mr Buchan's published works. That it concerns the land and history of Scotland, that it makes brilliant use of braid Scots dialect and that it enshrines many aspects, both admirable and contemptible, of the Scottish character are features that must give satisfaction to Mr Buchan's countrymen". Of the Buchan novels, Witch Wood was the author's own favourite and has been described as "a masterful tale of godliness in conflict with wickedness." C. S. Lewis wrote, "for Witch Wood specially I am always grateful; all that devilment sprouting up out of a beginning like Galt’sAnnals of the Parish. That's the way to do it". In The Interpreter's House, David Daniell notes that Witch Wood is tightly enclosed, with everything taking place under a heavy, black, suffocating pall of evil. Buchan's writing, Daniell says, "catches the obscene out of the tail of the eye, where it is most effective"; "Projecting his own favourite Scottish place, Broughton, back three centuries when the whole area was under forest, and doing it so convincingly, is a considerable feat. Doing it with such economic realism and keeping up a multiple pressure of plot makes it extraordinary". But Daniell's highest praise is reserved for the way Buchan presents "the ordinary people of the parish, the farmers who so represent the land that they are known by the names of their farms, the few cottagers, the herds and the elders and the children".
