Edwardian England. On the eve of his execution, Louis D'Ascoyne Mazzini, tenth Duke of Chalfont, writes his account of the events leading up to his imprisonment.

Louis's father, an Italian opera singer, died immediately after his birth; his English mother was heiress to the Duchy of Chalfont (uniquely able to pass by the female line), but her family disapproved of her marriage and she was disowned and disinherited. Impoverished and embittered, she ensured that Louis grew up well-versed in his ancestry. As a child, Louis fell in love with his classmate Sibella.

His mother's efforts to secure him a job in Lord Ascoyne d'Ascoyne's private bank are rudely rebuffed, and Louis finds employment at a drapery store. When his mother is fatally injured in an accident, her dying wish - to be buried in the d'Ascoyne family vault - is dismissed by the Duke; Louis vows revenge on the remaining d'Ascoynes between him and the Dukedom.

Louis lodges with Sibella's family. When Sibella snubs his marriage proposal and mocks Louis's lowly status, his fury grows and he begins to plot. Meanwhile Louis moves from the provincial drapery to another in the West End.

Louis's first victim is arrogant young Ascoyne d'Ascoyne, whose chance visit to the shop led to Louis's dismissal. Following his tormentor and his female companion to their riverside retreat, Louis arranges an 'accident', sending their boat over the edge of a weir. Louis's carefully phrased letter of condolence to Lord Ascoyne d'Ascoyne wins him a job at the bank.

Posing as an amateur photographer, Louis visits the village of young Henry d'Ascoyne, himself a photography enthusiast, and receives an invitation to Henry's home, where he meets his beautiful but abstemious wife Edith, who invites him to lunch that weekend.

Substituting petrol for the paraffin in the lamp in Henry's makeshift darkroom, Louis waits for Henry to disappear to indulge in a furtive drink. Louis escorts Edith to the funeral, where he encounters for the first time the entire surviving D'Ascoyne family.

Taking new lodgings, Louis begins an adulterous affair with Sibella - frustrated with her desperately dull husband, Lionel - while plotting to win Edith's hand. Meanwhile, four more D'Ascoynes are dispatched. The dreary Reverend Lord Henry D'Ascoyne sips poisoned port; the suffragette Lady Agatha D'Ascoyne is felled from her hot-air balloon with an arrow; Admiral Lord Horatio D'Ascoyne is involved in a (genuine) accident at sea; General Lord Rufus D'Ascoyne is blown up with an explosive jar of caviar.

Meanwhile, Louis is promoted at the bank and proposes to Edith, who delicately declines. Later, however, she reconsiders.

At Lionel's request, Louis visits his home. Lionel, drunk, explains he is bankrupt and begs for a loan. Louis refuses; Lionel, drunk, becomes angry, insulting Louis's father. Louis strikes him and they struggle; Lionel pulls a knife but is disarmed. Louis leaves. Later, Sibella visits. She lies that Lionel has discovered their affair and asks Louis to help her secure a divorce so that she can marry him. Louis calls her bluff and tells her he is engaged to Edith.

Louis and Edith are invited to Chalfont. Learning that the widowed Duke plans to remarry and produce an heir, Louis acts quickly, shooting the Duke while on a hunting trip. Lord Ascoyne D'Ascoyne, hearing the news, dies of shock.

As Louis is greeted by the staff of Chalfont, he is approached by a police inspector: he is arrested on suspicion of murdering... Lionel. At his trial in the House of Lords, Louis protests his innocence, insisting that Lionel killed himself. But thanks to Sibella's powerful testimony, he is convicted and sentenced to death. Edith, loyally, marries him in prison.

On the eve of his execution, Sibella visits Louis in jail: she notes that the emergence of a suicide note could still save him, and posits a scenario in which Edith meets an unexpected death and she becomes Duchess. Louis obliquely accepts the deal. All night, Louis waits for his pardon. At the last moment, it arrives. At the prison gates, he finds both Edith and Sibella waiting for him, amid crowds. A reporter enquires about his memoirs, and Louis realises he has left the incriminating document in his cell...