In
Edwardian England, Louis D'Ascoyne Mazzini, 10th Duke of Chalfont, is in prison awaiting his
hanging for murder, to be carried out the following morning. He writes an account of the events that led him to this point which forms the basis for the narration of the film told in
flashback. His mother, the youngest daughter of the 7th Duke of Chalfont, eloped with an Italian opera singer named Mazzini and was disowned by her family for marrying beneath her station. The Mazzinis were poor, but happy until Mazzini died shortly after Louis was born. Louis' mother brings him up among stories of the history of her family and tells him how, unlike most other
peerages, the dukedom of Chalfont can descend through female as well as male heirs. Louis's only childhood friends are Sibella Hallward and her brother, the children of a local doctor. When Louis leaves school, his mother writes to her kinsman, Lord Ascoyne D'Ascoyne, head of a
private bank, for assistance in launching her son's career, but is rebuffed. Louis is forced to work as an assistant to a
draper. When his mother dies, her last request to be interred in the family
burial vault at Chalfont Castle, is denied. Louis proposes marriage to Sibella, but she ridicules him, and marries Lionel Holland, a former school friend of her brother who has a rich father. Soon after this, Louis quarrels with a customer, Ascoyne D'Ascoyne, the banker's arrogant only child, who has him dismissed from his job. Louis resolves to kill Ascoyne D'Ascoyne and the other seven family members ahead of him in the
order of succession to the dukedom. After arranging a fatal boating accident for Ascoyne D'Ascoyne and his
mistress, Louis writes a letter of condolence to his victim's father, Lord Ascoyne D'Ascoyne, who employs him as a clerk. Upon his later promotion, Louis takes a
bachelor flat in
St James's, London, for assignations with Sibella, who is unhappy in her marriage to the dull Lionel. Louis next targets Henry D'Ascoyne, a keen amateur photographer. He meets Henry and is charmed by his wife, Edith. He substitutes
petrol for
paraffin in the lamp in Henry's
darkroom, with fatal results. Louis decides that Edith is fit to be his duchess. The Reverend Lord Henry D'Ascoyne is the next victim and Louis, posing as the
Bishop of Matabeleland, poisons Lord Henry's after-dinner
port. From the window of his flat, Louis then uses a bow and arrow to shoot down the
balloon from which the
suffragette Lady Agatha D'Ascoyne is dropping leaflets over London. Admiral Lord Horatio D'Ascoyne presents a challenge, as he rarely sets foot on land. However, by chance he conveniently insists on
going down with his ship after causing a collision at sea. Louis next sends General Lord Rufus D'Ascoyne a jar of
caviar which contains a bomb. When Edith agrees to marry Louis, they notify Ethelred, the childless, widowed eighth duke. He invites them to spend a few days at Chalfont Castle. When Ethelred casually informs Louis that he intends to remarry in order to produce an heir, Louis arranges a hunting "accident". Before shooting the duke, he reveals his motive. Lord Ascoyne D'Ascoyne dies from the shock of learning that he has become the ninth duke, sparing Louis from having to murder his kindly employer. Louis inherits the dukedom, but his triumph proves short-lived. Sibella's husband, Lionel, had made a drunken plea to Louis for financial help to avoid bankruptcy, but Louis turned him down. Lionel is later found dead and a
Scotland Yard detective arrests Louis on suspicion of murder. Louis elects to be
tried by his peers in the
House of Lords. During the trial, Louis and Edith are married. Sibella falsely testifies both that Lionel was about to seek a divorce and name Louis as
co-respondent and that no suicide note was found. Ironically, Louis is convicted of a murder he had never even contemplated. Louis is visited by Sibella, who observes that the belated "discovery" of Lionel's
suicide note together with Louis's promise to dispose of Edith would free Louis and enable them to marry, a proposal to which, facing the gallows, he is forced to agree. Moments before his hanging, the discovery of the note secures his release. Louis finds both Edith and Sibella waiting for him outside the prison, but is undecided which to choose. When a reporter tells him that
Tit-Bits magazine wishes to publish his memoirs, Louis suddenly remembers that he has left them behind in his cell (thereby providing the authorities with a complete
confession to the murders). ==Cast==