In Victorian Glasgow, Madeleine Smith and her family move into a new house in Blytheswood Square. Madeleine is being courted officially by Mr. Minnoch, but she is making secret assignations with a Frenchman, Emile L'Angelier, a poor clerk in her wealthy father's factory. He has humble lodgings and spends what little money he has on clothes to disguise his poverty, as a result of which he is behind with the rent. Madeleine shares a basement bedroom with her younger sister. When Emile visits her at night, he knocks softly on the window bars and then meets her outside in the yard, to which Madeleine has given him a key.

The Smith family go on holiday to a house overlooking the sea, and Emile takes the steamer to see Madeleine. They meet secretly in the garden. As the music from a village dance below the cliff rises up to them, they dance together.

Madeleine's father, a stern Victorian patriarch, grows impatient with her for not encouraging Mr. Minnoch's attentions. Back in London, on a snowy night, Emile and Madeleine meet in the maid's room. Emile tells Madeleine that he wants to be formally received into the household as a guest, rather than continue such furtive meetings, but Madeleine is afraid to let him meet her father. She visits Emile at his lodgings and begs him to elope with her, but he refuses. He wants to become part of her world, not to make her part of his. Madeleine is furious that he is really interested in her money, and, asking for the return of her letters, tells him she will not be seeing him again.

Madeleine dances with Mr. Minnoch at a ball and agrees to marry him. Emile lurks unhappily in the shadows. As preparations for the wedding begin, Emile comes to the house. When Madeleine again asks for the return of her letters, he threatens to show them to her father.

Madeleine sends a boy to the pharmacy for prussic acid, but the chemist refuses to sell it to him. Emile knocks on the front door of the house and is admitted to the drawing room by the maid, while the family are out. Madeleine serves him cocoa. He still wants to be invited formally as a guest, and asks Madeleine to play and sing for him. The next morning, at his lodgings, Emile becomes violently ill.

Madeleine visits the chemist to buy cough linctus, rose water - and arsenic. She signs the poison register and tells her cousin Charlotte that the arsenic is for her skin. Madeleine writes to Emile, asking him to come and see her, but he has gone away for a few days. When he returns, he is much better. That night, a heavy storm is brewing, but Emile goes out. On his way back, he is again taken very ill and a doctor is sent for. By the time he arrives, Emile is dead. The doctor is suspicious.

Emile's fellow lodger goes to see Mr. Smith. Madeleine is trying on her wedding dress. The friend thinks that Emile committed suicide and tells Mr. Smith that he had hoped to marry Madeleine. Mr. Smith did not realise that his daughter even knew Emile, and summons her to explain. Madeleine confesses that she knew Emile, wrote to him and planned to marry him. The friend tells her that Emile is dead, of arsenic poisoning.

Madeleine decides to go away but Mr. Minnoch catches up with her at the railway station and tells her that the police have been to see him. He persuades her to stay in town, and she gives him back his ring. When she returns to the house, the police arrest her for murder. As she is taken to court, crowds bay for her blood. She stands trial. Her defence counsel is brilliant and a verdict of 'not proven' is brought in.