Volume 1 of the book begins with a prologue set in "A.D. 186-" when Maurice Bertram, a boy of 7 holidaying in "Barling" (a fictionalised Brading) is thrown by his boisterous cousins into the doorway of "The Old House" and gets a bump on the head which is tended by the occupants, Miss Grace Dorrien and her mother Mrs Dorrien. Staying in the house, he is fascinated by Grace and her stories, and is disappointed to leave.
The action then shifts to "A.D. 185-" and Harwin Hall, somewhere in the north of England, where the immensely rich Sir Geoffrey Harbord is fulminating to his nephew and heir, Brinson Hythe. Sir Geoffrey has no reliable line of succession: Hythe's marriage has produced a sickly baby, Reginald, and an effete, mentally subnormal, poetry-spouting "changeling" called Pippin 1. He has disownered his real heir, his daughter Carrie (Grace's mother Caroline) for eloping to marry the soldier Clarence Darrien, and won't reconcile against Hythe's repeated advice.
Meanwhile, Grace is 19 and something of an embarrassment to her parents, Caroline and Colonel Dorrien, and has been variously nicknamed "Disgrace", "Scapegrace" and "Jack Dorrien" for her laddish behaviour (which includes earning money by dressing as a Spanish peasant and serenading passers-by outside the house). Her mother disapproves, but her delicate sister Laura admires her and her father is generally indulgent. When the family members take a holiday separately, he encourages her in her plan to visit her grandfather Sir Geoffrey.
With her faithful Scottish nurse Mursell, Grace goes to the Lake District and turns up unannounced at Hardwin Hall. Brinson Hythe refuses her entrance with the lie that Sir Geoffrey is out of the country. However, she doesn't believe him, and two days later returns to the house via the garden, where she encounters Sir Geoffrey and introduces herself. He is initially angry, but he's impressed by her audacity and charm, and invites her to stay. Hythe is not happy but remains polite. However, a series of unfortunate events befall Grace: first the simpleton Pippin shoots at her in the woods, then her rowing boat sinks due to a sawn-through plank. She suspects Hythe to be behind these attempts on her life, but nevertheless stays on, and even raises with Sir Geoffrey the prospect of her inheritance. Before this discussion can continue, she is called home by a telegram from her mother saying that Laura is very ill.
Laura isn't seriously ill, but the telegram was mainly intended to get Grace home. Her visit has repercussions both at Hardwin Hall, where Hythe is blackening her character to Sir Geoffrey, and at home with her mother's criticism of her general attitude to life.
A year later finds the family at Mentone, when Laura and the still-ill Grace are introduced to Mark Hilton, a young captain recovering from wounds received at the Siege of Lucknow (dating this segment early in 1858). At first they nickhame him "the Shadow" and "the Disenchantment" but he proves pleasant company as his health improves. On a trip to Monte Carlo, Grace dabbles in roulette, experiencing a bout of gambling fever that horrifies even her father, who has a similar weakness. Grace is attracted to Mark, but believes he is more interested in Laura, and they all go their separate ways.
Meanwhile, Hythe, who was also in Monte Carlo, uses the gambling episode as further ammunition to estrange Sir Geoffrey, as well as withholding Grace's letter to him. As a result, when Grace sends Sir Geoffrey a note on sighting him in London, she is despondent to get a deeply hostile reply. Shortly Grace comes of age, and her parents, with evident emotion, give her some documents to countersign in Double Indemnity fashion. Not long after, Colonel Dorrien dies in Calais, having been stabbed in a duel fought over accusations that he cheated at cards. Caroline Dorrien sends a letter to Sir Geoffrey begging his forgiveness: this too Hythe destroys.
Volume 2 begins with Grace, Laura and Mrs Dorrien arriving at Barling; a charitable relative, Lord Wotton, has offered them tenancy of The Old House. They are in poverty, it having been explained to Grace what documents she signed: Colonel Dorrien had tricked her into signing away much of her inheritance from his side of the family, in order to clear his debts. They settle in, accustoming themselves to the slow pace of life in the drowsy Barling. Grace - who has realised that Mark loves her, not Laura - continues to think about him, though he can't yet come home to England. A young naval officer on leave, Raymond Garenne, helps them renovate the house and becomes attracted to Laura, despite the news that she is dying of tuberculosis.
Sir Geoffrey, further goaded by Hythe, has a stroke. Laura's decline continues; and Grace, motivated by responsibility for her, refuses Mark's offer of marriage (due to his successful career in India, she would have to join him). This gesture proves futile when Laura elopes with Garenne. Her mother Caroline laments her as dead. Meanwhile Sir Geoffrey has realised that Hythe is intercepting his letters, and hands his servant a note before dying.
Volume 3, set in "A.D. 188- - 189-", finds Maurice Bertram, now a young physician working in nearby St Ann's, returning to visit The Old House, where he is captivated by Grace (remembering her as the sole offerer of kindness in his abused childhood) and sad to see the apparently deluded state of her mother Caroline. The latter, however, is relieved by the visit of Sir Geoffrey's servant, conveying the news that he forgave Caroline on his deathbed; she recovers her lucidity.
Grace and Mursell discuss various schemes to relieve their poverty, including Grace's failed attempt to become a writer, and a more successful line in selling produce from the garden. Time passes: Laura dies; Grace's mother dies; and Grace reads that Mark is now the venerable Lord Hilton of Khayala. Then one day, she receives a solicitors' letter announcing that, Brinson Hythe having died intestate and predeceased by his sons, she has inherited the entire Harbord fortune. She is one of the richest women in England. She curses the wealth that so poisoned her family's relationships, and declares to Lord Wotton (the only other person to know of the inheritance) that she won't accept it.
The Barling folk, intrigued by the enigmatic Miss Dorrien, begin to introduce her to their social circuit, where she makes a friend of a young woman called Margery (who seems strangely familiar to Grace, which is because she's Mark's daughter by his late wife). The increasingly successful Maurice Bertram declares his love for Grace, but she rejects him on grounds of age difference (he is 28, and she is 51) and brokers his marriage to Margery. Barling and its surroundings start to receive unnaccountable acts of philanthropy: funding for a clinic for Maurice, a convalescent home, a new pony for a carter whose old one died, and so on.
Grace becomes ill; Lord Hilton is sent for, and arrives just in time, so that she dies happy. At her funeral, she is revealed as Barling's anonymous benefactor. The now-married Margery and Maurice are bequeathed The Old House and a large fortune; to them and Lord Hilton it remains a lasting remembrance of her.
This is a sprawling novel that ideally ought to come with a family tree; MG has a bit of a 'teaser' approach to introducing characters, bringing them in one first-name terms then only later revealing the surnames that place them in context. I found The House of Hidden Treasure variable in quality - realistic story and insightful characterisation sitting alongside silly plot devices like Hythe's assassination attempts and Hilton's nick-of-time arrrival - but I found it overall very moving. This is chiefly because it's hard not to see a very unhappy Maxwell Gray behind the story, right from the introduction with its jaundiced authorial view of the end of the 19th century ...
... this very day—this very enlightened, hypercivilised day at the close of the century, a day so perfectly informed, so thoroughly schooled, as to have lost faith in virtue, honour, and truth; in decency, authority, and government; so surfeited with fairy tales of science, and rich in the long result of time, as to believe in nothing—save only steam, bacteria, natural selection, natural appetites, money, and ghosts.
... via its equally bleak description of writing as a profession ...
So Grace commenced author. For the next ten or fifteen years her handwriting embellished the waste-paper baskets of many editors, and distressed the sight of many publishers' readers. Some poems found their way into print, but she never saw them. Once a sketch of village life was printed in a magazine that went bankrupt, and paid nothing. Once she received a guinea for a spirited story of five thousand words; once she had ten shillings for a descriptive essay. But she reaped a rich harvest of bitter disappointment, and sickening heartache, of hope long deferred and foiled at last; and suffered much weariness of brain and hand from labour that profited nothing.
There is much pain and weariness and disappointment, and there are many wearing and exhausting occupations in this work-a-day world; but there is nothing that crushes life from the heart and health from the body, like writing on without success. Efficiency in all other crafts and trades can to some extent be gauged; but there is no standard by which unpublished literary work can be measured, while the one person who sees it, is the least capable of appraising it. So at last Grace Dorrien gave up the profession of author.
... to the overall resemblance of Grace's situation to that of Maxwell Gray (at the end of the book the ailing Grace is much the same age as MG when she wrote it). MG was at the peak of her fame as an author (see Google Books N-gram Viewer) and solvent enough to have moved to fashionable Richmond; but well into middle age and crippled by chronic illness.
Geographically, The House of Hidden Treasure is partially an Isle of Wight novel; its Barling has detailed descriptions matching Brading, along with other locations in MG's fairly consistent mythos (such as Newport as "Oldport", Sandown as "Sandyknowe", and St Helens as "St Ann's"). There's no house matching the location of "The Old House", which is said to be directly opposite the Bugle (a real inn in Brading), although its appearance and age match the Tudor building nearby. Hardwin Hall's location isn't clear: various descriptions place it as "on the way to Keswick" from the south, on land with woods and fields on the west coast of England in sight of the sea from its turret, with Kendal as the railway station, and ten miles' ride from a country town with a cathedral (which can only fit Lancaster). If I had to make a guess, I'd say it was modelled on Levens Hall in south Cumbria. The French health resort of Menton - another regular venue in MG novels - also makes an appearance, under its old name of Mentone.
The House of Hidden Treasure is on the Internet Archive (ID househiddentrea00graygoog).
1. Think Basil Fotherington-Thomas in the Molesworth series. I imagine Sir Geoffrey to resemble Finis Everglot.