In 1855, a clever young lawyer called Delamayn finds a loophole in the Irish marriage laws which enables John Vanborough to annul the marriage that stands in the way of his parliamentary ambitions. The abandoned wife, Anne Silvester, lives with her old friend, Blanche, married to Sir Thomas Lundie, together with their respective daughters, also called Anne and Blanche.

Twelve years later Vanborough, and Anne and Blanche senior are all dead. Sir Thomas has remarried but dies shortly after, when the title and estate pass to his likeable brother, Sir Patrick Lundie, a retired lawyer in his seventies. Sir Thomas is survived by his second wife, the overbearing Lady Lundie, now responsible for the two daughters. The lawyer, Delamayn, has succeeded in life and become Lord Holchester with two sons, Geoffrey and Julius.

The main action of the story begins in 1868 at Windygates, the Perthshire home of Lady Lundie. Blanche is now 18 and Anne, acting as her governess, 25. Visiting them are Sir Patrick; Arnold Brinkworth, engaged to Blanche; and Geoffrey Delamayn, the school friend who once saved Arnold's life. Geoffrey has promised to marry Anne but is now trying to break the secret engagement because he will be cut out of his father's will. Anne persuades him to accept a secret marriage and knowing a little of the Scottish law arranges to go to a lonely inn at Craig Fernie where she will announce that she is waiting for her husband. If Geoffrey arrives and openly asks for his wife, the presence of witnesses will validate the marriage according to Scottish law.

Geoffrey receives a telegram that his father is dangerously ill and must go to London with his brother. He therefore asks Arnold to deliver a message to Anne at the inn, carefully telling him not to use his own name but merely to ask for his 'wife'. Arnold reluctantly agrees but makes Geoffrey put the message in writing. The note, stating Geoffrey's intention to marry, is written on the back of a letter from Anne signed 'your loving wife'. Arnold arrives at Craig Fernie and to preserve Anne's reputation pretends to be her husband in front of the Landlady and the head waiter, Bishopriggs. Anne reads the letter which is subsequently stolen by Bishopsriggs while Arnold innocently continues the deception, sleeping on the sofa and leaving early the next morning.

Lord Holchester recovers but on his sick-bed remembers he was responsible for the infamous treatment of Anne's mother and asks Julius to help her if she should ever need assistance. At the same time Lord Holchester will only accept Geoffrey back into the family if he makes a respectable marriage. Julius and his mother have in mind the young widow, Mrs Glenarm, whom Geoffrey will meet on his return to Scotland.

Anne is worried about her marital status and without disclosing Arnold's name, has Blanche consult Sir Patrick. Geoffrey overhears the opinion that the marriage is ambiguous, but that two people are legally bound in Scotland if they pretend to be married. He therefore adopts the view that Anne is technically married to Arnold and he is free to marry Mrs Glenarm. By this time Geoffrey has agreed to represent the South in the great foot-race, a national athletic event against the North, although a medical guest in the house, Mr Speedwell, warns that the exertion would endanger his health.

Anne visits Blanche at Windygates and meets Geoffrey who declares he cannot now marry her as she is already married to Arnold. In a state of shock, she deliberately leaves her friends and hides in Glasgow where she consults two different lawyers. They give conflicting opinions as to whether she is married and she collapses under the strain with a miscarriage.

Ignorant of Anne and Arnold's position, Sir Patrick brings forward Blanche's wedding to Arnold. The couple are recalled from their honeymoon in Switzerland when Sir Patrick hears from Anne, who reveals Arnold's predicament and the conflicting legal opinions. Anne meanwhile recovers the stolen letter from Bishopriggs and tries to convince Mrs Glenarm of Geoffrey's prior marriage. Lady Lundie deduces that Arnold was the man at Craig Fernie and mischievously persuades Blanche to leave Arnold for her house in London where Mrs Glenarm is also staying. Sir Patrick must prove the Craig Fernie marriage is invalid to safeguard Arnold's position with Blanche, whereas Mrs Glenarm wants it confirmed so that Geoffrey is free to marry her.

Geoffrey has rented a secluded house in Fulham, left to Lady Lundie's former cook Hester Dethridge, an elderly woman who is mute after a traumatic marriage to a drunken husband and communicates only in writing. Against medical advice Geoffrey completes his intensive training for the foot-race. On the last lap of the four-mile course he collapses, unable to continue. He is abandoned by his friends because they lost money on the race and is taken home by his trainer, Perry

Two days after the race, all parties attend a family conference with their lawyers. Geoffrey refuses to accept Anne as his wife and Blanche will not return to Arnold until the ambiguity of the Craig Fernie marriage is resolved. Anne, despite knowing she will have to return to Fulham, bravely produces her letter which according to the Scottish law of consent confirms her marriage to Geoffrey.

Following the death of Lord Holchester, Julius has inherited the family title and visits Geoffrey to proposes a separation. In return Julius will honour an unsigned codicil to their father's will, leaving Geoffrey a secure income. Geoffrey, however, having no grounds for divorce, intends to kill his wife. Pretending he will be a good husband he refuses his brother's offer and keeps Anne prisoner in the house.

Geoffrey has always been unnerved by the silent and withdrawn Hester Dethridge. The secret of her own terror of him is revealed in a written confession of murder. Years before, when she complained to the police that her husband ill-treated her and stole her money, they were unable to help because the law gave a married woman no right to her own earnings. She determined to kill him as the only way to end her misery. While her husband lay in a drunken stupor, she suffocated him through the partition wall of his locked bedroom after carefully removing and replacing the lathe and plaster. Hester was not suspected of the crime but now sees visions of a second self urging her to kill again.

Geoffrey blackmails Hester into helping him murder Anne by the same means but at the last moment she sees the apparition again and attacks Geoffrey. In the same instant he suffers a fatal stroke. Hester is confined in an asylum while Anne, free of Geoffrey at last, supplants Lady Lundie by marrying Sir Patrick and becoming the new female head of the family.