South East England freezes under the coldest winter on record, as Mrs Anna Bünz, a German folklore enthusiast who emigrated at the start of the war, drives from her Worcestershire home to the tiny village of Mardian, in search of "The Dance of the Five Sons", a folkloric survival incorporating in uniquely rich profusion all the elements of English Morris, sword dance, guising and mumming.
Andersen's granddaughter Camilla, a young actress, is also staying at the pub, hoping to reconnect with the family who rejected her mother Bessie for marrying outside her class and community.
Hovering uncomfortably around this class hierarchy is an affably boozy ex-RAF hero, Simon Begg, who runs the local garage and also has a key role in the mumming play.
His investigation draws together the story's fundamental fascination with English folkloric traditions and the changing world that is impinging on Mardian's rigidly class-oriented life, as represented by William Andersen and Dame Alice.
Dr Lewis writes that the novel's background was very carefully researched, as was Ngaio Marsh's habit, and that "her library in Christchurch contains several reference books on folk dance and ancient customs."
Ngaio Marsh was characteristically self-doubting and modest as she submitted the manuscript to her London agent Edmund Cork: "I'm in such a stew over it, not knowing if it's deadly dull or passable."
And: Off With His Head "is an unusual novel and deserves better than categorisation as simply another piece of formula fiction... [T]he enduring nature of the ancient village and its pagan rites is ultimately more memorable than the routine task of identifying a murderer."
Reviewing Marsh's 1955 Scales of Justice (which preceded Off With His Head), the New Statesman critic acknowledged her "magnificent workmanship" but found her books "often heavily loaded with crudely snobbish class consciousness".
Marsh biographer Margaret Lewis refers to a filed BBC memo rejecting a radio dramatisation of Scales of Justice as suffering from "appalling snobbishness".