Phryne is having cocktails with Lucy, Lady Wessex who has asked for Phryne help as she is being blackmailed by a man who she had a fling with. The man is threatening to tell her husband and has a picture in a locket of Lucy and him together. She said James was unhappy as the locket was his wedding present to her that he had bought out from Glasgow. She says she can't raise the money as she has nothing to sell without raising suspicion.
Phryne asks about a paining in the hallway and Lucy says it is St Mungo the patron saint of Glasgow. St Mungo had helped a women whose husband had found out she had given her marriage ring to her lover. The Husband had killed the lover and threw the ring into the river and told the wife she must present herself wearing the ring at dinner that night or she too would be killed. St Mungo called up a salmon and took the ring out of its mouth and gave it back to the lady.
Phryne asks Lucy about her lover and she says he is a medical student named Percy and friend of her brother. They had been seeing each other for three months but he asked her to leave her husband and she said no. Then he started demanding money.
Phryne says she has an idea. They both dress for dinner and go downstairs. After dinner with a bunch of bright young things a young man asks Phryne to dance. Phryne refuses but says she will talk to the man for a while. He introduces himself as Gerald, Lucy's brother. He says he is at university studying art as their father is rich so he doesn’t need a profession. He says a lot of the young men at uni are not rich and need to work to make money like Percy and Tommy. Tommy is a bouncer at Montpellier a place the boys go to play cards. Phryne asks Gerald to take her there and he says he will the next night.
The next night they go to the Montpellier a place of squalor. The go to a side room where the card game is going on and Phryne notices a young man who she suspects is Percy. She introduces herself and it is Percy so she asks him to cut her in to the poker game. Phryne starts losing hand after hand until she has not money left. Then she offers her virtue for a piece of jewellery. Percy agrees and she tells the others to leave the room. Phryne asks to see the stake and Percy asks her why she is doing this she says a whim. Percy notices Phryne is a lot more composed and seems less drunk than earlier and that she had pulled to cards out of the deck that were not the ones he had stacked. Phryne puts her cards down, a royal flush and Percy accuses her of cheating. She admits he did but said he did too and only noticed her marked cards because her marks were in different places to his. Percy reaches to grab Phryne but she pulls her pistol on him. Phryne tells him to let her go and she wont tell his friends about his cheating or Gerald about Percy's relationship with his sister. Percy agrees and Phryne leaves.
The next morning Phryne asks Lucy about that mornings fish delivery. She then plants the locket in the fish without Lucy knowing. Later that morning a yell comes from the kitchen and Lucy goes to find out what is going on. The cook comes out with the fish with the locket gleaming from its belly. She says she recognised the locket as the one Lucy had lost overboard. Lucy's husband tells her to wash the locket and put it back on and she hugs him happily crying. He tells Phryne he too remembers the story of St Mungo and she raises her glass to him.