Plot summary
When their father dies, Doctor Thomas Thorne and his younger brother Henry are left little from the estate. Thomas begins to establish a medical practice, while Henry seduces Mary Scatcherd, the sister of stonemason Roger Scatcherd. When Roger finds out that Mary has become pregnant, he gives Henry a thrashing, inadvertently killing him. Roger is found guilty of manslaughter and sentenced to six months in prison.
While her brother is incarcerated, Mary gives birth to a girl. A former suitor offers to marry Mary and emigrate to America to start a new life, but not if she keeps the baby. Doctor Thorne persuades Mary to accept the offer, promising to raise his niece. Mary tells Roger that the baby has died. Thorne names the child Mary Thorne, keeps her parentage secret, and arranges for her to be looked after on a nearby farm and later to attend a boarding school.
When Thorne becomes the family doctor to the Greshams, the preeminent family of the county, he also becomes advisor to the head of the family, squire Frank Gresham. About this time an heir is born at Greshamsbury Hall to much celebration. The aristocratic mother, Lady Arabella, engages a wet nurse for her son, also named Frank. However, the girl proves to be too fond of brandy. Thorne knows that Mrs Scatcherd had been left in dire financial distress by her husband's imprisonment, so he arranges for her to take over the role.
Time passes, and 12-year-old Mary Thorne came to Greshamsbury from her school in Bath. Soon she endears herself to the Greshams. Dr Thorne feels he has to inform Frank Gresham, in strictest confidence, of Mary's background to allow him to decide if he would object to her associating with his family. Lady Arabella some qualms but the squire overrules her.
Mary's maternal uncle, Roger Scatcherd, rises from humble beginnings to great wealth and a knighthood through his prodigious skills, not only as a mason but also as a contractor. Doctor Thorne, who visited Scatcherd in prison and befriended him, becomes his financial advisor. Thorne persuades Scatcherd to lend increasing sums to Frank Gresham, who had impoverished himself by running for Parliament three times. Eventually, much of the Gresham estate is put up as collateral for these loans.
As young adults, Mary and the younger Frank Gresham fall in love. However, his parents need him to marry wealth. As Mary is penniless and of suspect birth, such a marriage is inconceivable to his mother and to her relatives, the aristocratic de Courcys. They wish Frank to marry the eccentric 30-year-old heiress Martha Dunstable instead. Frank reluctantly visits Courcy Castle in order to meet Miss Dunstable, and they become friends. He foolishly and playfully proposes, but she wisely demurs, knowing that he does not love her.
Sir Roger Scatcherd is a chronic drunkard, and Doctor Thorne tries in vain to get him to curtail his drinking. In his will, Scatcherd leaves the bulk of his estate to his only son, the dissolute Louis Philippe. However, he leaves Doctor Thorne in control of the inheritance until Louis Philippe reaches the age of 25. Should Louis die before then, Scatcherd stipulates that the estate go to his sister Mary's eldest child. Thorne, knowing that Scatcherd is thinking of the children Mary had in America, is forced to divulge Mary's parentage, but Scatcherd leaves the will unchanged.
Roger Scatcherd eventually dies of drink. The son proves just as much an alcoholic as the father, and his weaker constitution quickly brings him to the same end. After consulting with lawyers, Doctor Thorne confirms that his niece Mary is the heiress, now wealthier than even Miss Dunstable.
Unaware of these developments, the still resolute Frank finally persuades his doting father to consent to his marriage to Mary. When all is revealed, all of his relations heartily congratulate him.
