Based on Roy Horniman's novel, a young man murders several members of a family so that he can inherit a dukedom.
Louis Mazzini (Dennis Price) prepares to be executed and tells his story. His mother Louisa d'Ascoyne (Audrey Fildes) elopes with an Italian and is spurned by her aristocratic family. When Louis is born, his father dies. His mother is poor but could inherit the dukedom. Louis works for a draper, and his mother dies. Louis vows revenge and watches his family tree.
Louis asks Sibella (Joan Greenwood) to marry, but she rejects him. Louis sells cloth to a d'Ascoyne (Alec Guinness), insults him, and loses his job. Louis follows him and his girl on a river and causes them to drown. Sibella tells Louis that she is engaged to Lionel Holland, but she asks Louis to kiss her.
Louis goes after young Henry d'Ascoyne (Alec Guinness) with a camera and meets his wife Edith (Valerie Hobson). Sibella marries Lionel (John Penrose). Louis visits Henry and Edith, and Henry dies in the darkroom. At the funeral Louis sees his impediments. The Duke's heir (Alec Guinness) hires Louis as his private secretary. Sibella visits Louis, says Lionel is boring, and kisses him. Louis pretends to be a bishop and poisons the elderly Reverend Henry d'Ascoyne (Alec Guinness).
Lady d'Ascoyne (Alec Guinness) is a suffragette, and Louis shoots an arrow into the balloon from which she is dropping leaflets. Admiral Horatio d'Ascoyne (Alec Guinness) goes down with his ship, and Louis sends General Rufus d'Ascoyne (Alec Guinness) gunpowder in caviar that blows up in his face. The Duke's heir tells Louis that only he stands between Louis and the Duke, and he makes him a partner.
Louis courts Edith, but she says she will not remarry. Louis admits to Sibella that he murdered them all. The Duke's heir has a stroke. Edith visits Louis and agrees to marry him after three months. Louis is summoned by Lionel, who is tipsy and says he is bankrupt and may kill himself. Louis feels insulted, and they fight. Sibella tells Louis that Lionel is divorcing her. She wants to marry Louis, but he says he is engaged to Edith.
Louis and Edith are invited to the castle, and Duke Etherel d'Ascoyne (Alec Guinness) shows him around. Etherel says he is getting married and hopes for a son. Louis goes shooting with him, and Etherel has a trapped poacher thrashed. Louis leads Etherel into a trap and takes his shotgun. Louis shoots him so that it looks like an accident. The old ninth Duke dies of shock.
Duke Louis announces he will marry Edith, but Inspector Burgoyne arrests him for the murder of Lionel. Louis is tried before the House of Lords. Sibella testifies against him. Edith says she wed Louis in prison, and Louis is cross-examined. The Lords find him guilty. Sibella visits Louis and says Louis' suicide note could be found if he was willing to murder Edith. Louis completes the writing of his story. At the last moment he learns that the suicide note was found. Louis is released, but he realizes that he left his memoirs in the cell.
In this black comedy narrated by a sophisticated murderer
his calm demeanor and polite manners are contrasted to his violent
deeds. Perhaps it symbolizes the demise of the British aristocracy
in a democratic age.