It sheds light on English life and childhood in the First World War, through a good-natured pre-adolescent female character, presented in detail, and a realistically written time-slip narrative.
The book begins with her arrival unaccompanied by train, to find that her "holiday" aunt's uncongenial children have caught whooping cough.
Jessamy has to be farmed out for the summer to Miss Brindle, the childless caretaker of an empty Victorian mansion: Posset Place.
Jessamy is taken aback by the old Miss Brindle, who in turn is wary of children: "I daresay you won't mind being treated like a grown-up person.
She is discovered by a parlour maid, Matchett, who is up and in street clothes suspiciously late, and asks crossly what she was doing in the cupboard, "'I don't quite remember,' Jessamy heard herself say slowly.
In the present, she is a brave, well-meaning and intelligent enough girl, but isolated and deprived of love and companionship, not to mention adventure, and wishing she could go to boarding school.
The grandfather Mr Parkinson, owner of Posset Place, takes Jessamy, his grandson Kitto and the groom William Stubbins to an auction, where he buys a medieval book of hours for the large sum of £300.
The eldest boy Harry, everybody's favourite, then returns from Oxford, set upon joining the army instead of completing his final year, and burdened by debts.
On arrival in the earlier Posset Place, Jessamy promises Matchett, who is up late at night, not to betray her love affair.
Disobeying Mr Parkinson's orders to the children never to climb the tree again, Jessamy goes up to retrieve the knife, but is caught in the act.
The door of the cupboard shuts behind Jessamy and she finds herself back in the present, again wearing her dressing gown and holding not a candle, but her torch.
She succeeds, picks up the strands, and becomes skilled at soothing Billy, the baby boy of Matchett, by then Mrs Stubbins with a husband away in the army.
She manages to crumple the note and tuck it into the mouth of a tiger hearth rug in the drawing room, but Mrs Stubbins chases her up to the schoolroom, where Jessamy hides in the cupboard – and promptly returned to the present.
There is one more set of marks in the cupboard, dated September 10, 1916, but Jessamy, to her sorrow, fails to slip back in time on that day.
(p. 159)[2] Kirkus Reviews considered the story to be "tellingly told", noting that it had "so much of the intrigue and fantasy that a young girl loves to daydream over.
[4] A German translation entitled Der Spuk im alten Schrank (The spook – or mischief – in the old cupboard), by Marie-Louise Dumont and illustrated by Sita Jucker, also appeared in 1968.