This week's Miss Fisher's Murder Mysteries begins with women dressed as nymphs dancing about on the lawn, as Prudence watches from the porch. She's joined by Dr. Haydon Samuels (Philip Quast), who she's allowed to use her place as a sanitorium while he helps her deal with the trauma of losing Arthur.
When one of the nymphs, Betsy, runs in and takes to her bed, the lights flicker, and next thing you know, the poor child is dead of electrocution. When Fisher and Dot arrive, they are shocked by the state of things. Prudence says it was just supposed to be Samuels and a few girls, then Dr. Allen Perkins (Damon Gameau) and secretary, Harriet Edwards (Arianwen Parkes-Lockwood) turned up.
Fisher: Perkins Percussor! I think all our murder weapons should be labeled.
Jack is upstairs when Fisher arrives, and sizes up the appliance in question as a vibrator (ahem, electric massager"), perhaps used by one of the doctors, who left behind half a collar stud. The new constable, Neville Martin (Henry Hammersla), finds himself run over, first by Dot, then by Fisher, who orders him to search for the missing appliance. (Collins is on leave after last week's attempted resignation.) In the drawing-room, Dolores (Jane Allsop) tells Dot Betsy was consumed with lust. Meanwhile, Fisher and Jack sit down with the doctors just as Harriet's brother Eric (Gareth Reeves), Samuels' solicitor, shows up. Samuels insists it's a coincidence, but Edwards interferes with questioning anyway.
Irritated, Fisher finds Jemima (Laura Gordon) crying in the hallway. She calls Betsy her BFF, like a mother to her, but before she can get much more, Harriet rounds the girl up. When Fisher asks Prudence if there have been incidents, she notes Jemima tried to push Betsy down the stairs. Prudence also admits Samuels was treating Arthur before the poor lad died in his sleep. At the autopsy, Mac notes the electrocution seems to have been through the hand, but the fingers were broken by someone else, probably trying to get the missing appliance out of her hand. She also tells Fisher Wilbur Littleton (Rod Mullinar), the current head of the medical association, has accused Samuels of inappropriate behavior.
When Fisher and Jack meet Littleton, he says Samuels was called before the board for malpractice when a former patient complained about "the Percussor" Perkins developed. He has also lost a few patients to Samuels, who "picked his targets," including Betsy. When he hears Betsy is dead, he becomes alarmed. His daughter, Jemima, is one of Samuels' patients, persuaded by Betsy to join the sanitorium. He calls his daughter "obsessed" with Betsy. Littleton's wife died when Jemima was very young, and she's been latching on to mother figures ever since.
A search of the office turns up a photo of Samuels in Harriet's desk, a fern leaf on the floor by Perkins' chair, and several "Deceased" files. Fisher tracks down the plant in Prudence's greenhouse and finds the "Perkins Percussor" behind it. Perkins admits he hid it in a panic when he found Betsy, thinking it had gone faulty. Samuels insists it should have been locked away. Fisher confronts him with the empty "deceased" file of Victoria Broadford. He says the family requested it "for legal reasons." Just then, Edwards arrives, demanding his client's release or he'll sue. Samuels tells him to cool it; the last this his reputation needs is "another court case."
Mac looks into the Broadford case and discovers the family accused Samuels of poisoning her. Meanwhile, Martin turns up the Broadfords also accused Samuels of conning Victoria into leaving him all her money, something Betsy also did before passing. Harriet insists the women are doing it of their own accord. But when Phryne sees Prudence donated a thousand pounds, she panics, since her aunt is so tightfisted. But Prudence roars back her financial choices are her own.
Littleton comes to take Jemima away, having booked a hysterectomy. Samuels admits all his patients face this without his protection, and Betsy would have been castrated to boot. Fisher rechecks the office and finds a hidden key under Perkins' desk and grabs the blotter paper, which Butler finds is a letter about a patent. Jack picks up Harriet, who confirms there was money missing, and Perkins wanted to patent his device and sell it commercially. Perkins admits he took the money, but he was owed it. Besides, he wouldn't have tampered with his creation if he was trying to sell it, nor did he know about Betsy's bequeathment.
At Prudence's, Littleton arrives to shut down Samuels' work, sending the women into sobs of horror. Meanwhile, Harriet starts screaming because someone strangled her beloved birds. Samuels hypnotizes her into calm. While under the influence, she starts to tell him something. Assuming she's going to confess her love, he feeds her a bit of one of his tonics and sends her to her room. Outside, Jemima, blaming herself for her father shutting the sanitorium, has climbed to the top of Prudence's house, threatening to jump. Fisher climbs out on the ledge to talk her down. Jemima tells her Harriet gave Betsy the Percussor because Betsy blackmailed her into it.
What Betsy knew (and Jemima has repressed) may not matter. Harriet's dead. Mac says it's the same poison that killed Victoria. Harriet also had the missing half of the collar stud, proving she knew who did it. So Fisher decides to let Samuels use hypnosis on Jemima to see if they can bring it out. (This is not how hypnosis works.) Jemima says they saw Harriet having sex with someone. Fisher realizes there are only two men who it could be, so she sets a trap, and has Bert offers Edwards the missing pin. The lawyer thoughtlessly confirms it's his.
Eric and Harriet were committing incest, something she never thought was "wrong" until meeting Samuels. When Betsy revealed she knew by blackmailing Harriet into the percussor, he tampered with the device to kill her. When Harriet tried to tell Samuels in the drawing-room with Eric in earshot, he killed her too. He made it match the Broadford case, to frame Samuels. Unable to continue his work in Australia, Samuels decides to restart his practice in Switzerland, taking Jemima to protect her from her father. But before he goes, he and Fisher get together to stage a celebration of Arthur's life, and Prudence finally lets herself grieve.