Victorian Brighton. The Brighton Herald reports on a murder trial, marking the first appearance of new Public Analyst Edward Sutton. The female defendant is found guilty and sentenced to hanging, thanks to Sutton's evidence.

In his pharmacists shop, Sutton discovers his son David is carrying on an illicit correspondence with his sister's friend Mary. Later, he speaks to Mary's father, and instructs David to put aside thoughts of marriage. Meanwhile, Sutton rejects his eldest daughter Victoria's request to train to become professional singer and, when younger daughter Peggy thanks him for the guinea pigs he gave her for her birthday, he coldly informs her that they were not a gift but for use in his experiments.

Defiantly, David visits a pub on the seafront. The landlord's wife, Pearl, is having a none-too-secret affair with lowlife Dan Powell, infuriating her husband, Joe Bond, and Dan's lady friend, Louise. In the back room, a drunken Joe demands that Pearl stop seeing Dan. Back in the bar, Pearl gets into a fight with Louise, until Dan breaks it up. Pearl storms outside, meeting the tipsy David. She is cold with him, until Dan appears with Louise, when she accepts David's invitation to take a walk.

Next day, Victoria and Peggy spy a poster advertising the famous singer Madame Patti. Hoping for an audience, they wait outside the Royal Pavilion and when Madame Patti leaves, Victoria, urged on by Peggy, sings. Madame Patti is impressed. Later, Peggy and Victoria talk excitedly about Madame Patti's offer to arrange an audition for Victoria in London. But realising she can't afford the fare, Victoria is crestfallen. When her father confiscates Peggy's pocket money as punishment for feeding the guinea pigs, Peggy withholds her church collection money to help Victoria get to London.

In an oyster bar, Dan flirts with the manageress, Mrs Webster, telling Pearl that if she too were a wealthy widow he would be completely hers. Back at the pub, Pearl finds a furious, drunken Joe. They argue; he pushes her to the ground and she cuts her hand on a glass. Leaving, she runs into David, who takes her to the pharmacy and dresses the wound. Keen to impress, he shows her the poisons and tells her that strychnine poisoning is almost indistinguishable from lockjaw; in his father's recent murder trial, the crime was uncovered only because there was no wound to allow the infection to enter the victim's body. While David fetches her a drink, Pearl surreptitiously fills her handkerchief with strychnine.

While Joe lies drunk and unconscious on his bed, Pearl cuts his hand with a razor, then locks the door. Three days later, Pearl empties the powder in Joe's drink and watches, horrified, as the poison takes effect. She then pounds on his locked door, screaming for help. Dan, who is entertaining Mrs Webster in the bar, runs to her aid, and he and the barman break down the door. Joe is pronounced dead and a shocked Pearl is helped away. A doctor diagnoses lockjaw.

Victoria is awarded a scholarship to the Royal College of Music; her father forbids her to go. David visits Pearl after the funeral. She is frosty, and when he suggests they go out some time, she laughs. Pearl's relationship with Dan is now in the open.

Louise discovers that Joe didn't, as Pearl claimed, cut his hand. She becomes suspicious and tells the police. The body is exhumed. Panicking, Pearl admits the murder to Dan. He suggests a visit to the public analyst. Pearl visits Sutton and insists that David is in love with her; it was his idea to poison Joe. If Sutton confirms the diagnosis of lockjaw, it will remain a secret. Sutton is unconvinced, but determines to question David.

Satisfied with David's story, Sutton visits Pearl and points out the flaw in her plan: he cannot participate in a case involving his own son - her fate is sealed. Pearl is shattered and, when Dan abandons her, she leaves the pub and, in a daze, throws herself over the seafront.

Later, the Brighton Herald reports the wedding of David Sutton to Mary Truscott, and comments on David's sister Victoria's successful singing career.