This week's Miss Fisher's Murder Mysteries cold open is the most gruesome yet, featuring a female factory worker thrown into a machine. It's horrible enough to upset the unflappable Dr. Mac, who turns up at Phryne's house first thing the next morning, traumatized by what she saw when she answered the emergency bell. The victim's name was Daisy Murphy, and Mac insists her death no accident.
Miss Fisher: I was a very charming freight train.
Fisher is surprised how well-known Mac is at the factory when they arrive on the scene. Mac says she's been tending to the owner, Roderick Gaskin (Andrew Blackman). She was on site doing his regular injections when Daisy's death was called in. Collins is there when they arrive, sick to his stomach over what he's seen. He's also signed paperwork for Gaskin, declaring this an accident. Mac wants to push it, but Gaskin sits on the hospital board and is willing to threaten her job if the two don't leave at once. Robinson is also ready to write things off until Fisher points out that if Gaskin's timeline is correct and Daisy died at the beginning of the shift, 8 a.m., how had the blood splatters completely dried by half-past nine?
Robinson agrees to investigate. Gaskin blusters, trying to get his sister, Joyce (Alison Whyte), out of the room. It's obvious why, she takes charge of answering questions as soon as Robinson starts asking, and none of her answers line up with his claims. When Fisher points this out, Robinson asks her to wait outside, giving her a perfect opportunity to do some sleuthing undisturbed. Then the foreman, Ted Coglan (Neil Pigot), catches her taking pictures of the scene. Her feminine act doesn't fool him, and Gaskin threatens to have her arrested. Robinson agrees there is cause to doubt Gaskin, but he'll have to investigate without her.
Fisher's photos prove the safety guard that should have prevented Daisy from falling in was replaced after the accident. Mac says she knows most women are told to remove them because "it slows them down." But that's negligence, not murder, even if Daisy was threatening to be a whistleblower over some mysterious goings on at the factory. Aunt Prudence stops by to yell about Phryne's near-arrest. She admits his money to the hospital is buying respectability, but she can do nothing to protect Mac from his accusations of her being "unchristian." With Fisher banned from the premises, she and Mac hatch a plan to send in a new face to investigate: Dot.
Dot gets hired as a tea server, making friends with Hetty (Caroline Brazier), the head "Tea Lady." They serve everyone daily, which is how Dot discovers Joyce has a private ledger. It turns out to be for extra hours/wages, Joyce is having the girls work off the books. It also shows Daisy worked one of these secret 6-10 p.m. shifts the night she died. When Dot spies Coglan passing extra pay to Lorna (Joanne Trentini), who is listed in the ledger, she asks how one gets in on the racket. But before Dot can get answers, Gaskin's body flies out of the top office.
Robinson and Collins are stunned when Dot turns out to be their "Tea Lady" witness. Joyce and Coglan both claim they were in the corridor, and Gaskin was alone when he fell. Joyce attempts to pin suspicion on Dr. Mac, who was in that morning doing Gaskin's injections. But Mac's alibi holds up, she left the factory at 8:50, saw her first patient at 9:05, and it's a 15-minute drive from one to the other. That's why Fisher is floored when Prudence shows back up later, announcing Robinson arrested Mac, and Joyce is demanding the board suspend her.
The evidence is pretty stark — Gaskin's medicine was mixed with bleach. A doctor would know it would take 10-15 minutes to induce a heart attack, perfect alibi. Horrified, Fisher gets Cec and Bert to go on a mission with her to see what happens at the factory after-hours, to prove Joyce is using the factory to run a side business. When Robinson turns up the next day with Fisher's evidence, Joyce points out she runs the factory, why would she rob herself? But Robinson thinks she killed her brother to get him out of the way, and the droplets of bleach in the wastebasket suggest he's right.
But when Dot mentions to Hetty that Mac's been arrested, Hetty goes to Robinson and spills everything. Mac was secretly helping the factory girls get family planning, access to midwives who do abortions. As for Daisy, the girl was Mac's lover. Fisher is not surprised Mac is a lesbian, but heartbroken she didn't disclose the relationship. But Mac still isn't in the clear until Collins finds the tampered medicine vial in the trash, proving Mac didn't mix the bleach herself.
But Joyce didn't do the tampering. She was having an affair with Coglan and left behind her desk key. Fisher realizes the one people who could walk into Gaskin's office unquestioned is the Tea Lady, Dot discovers there are love letters to Daisy in Hetty's locker. Hetty admits she killed Daisy in a lover's quarrel because Daisy was leaving her for Mac, and then framed Mac for Gaskin's murder. Now she's going to kill Dot too. Fisher and Robinson arrive before Hetty pushes her into a machine. Fisher saves Dot by taking an ax to the electrical panel, shocking herself silly in the process. Mac is innocent, and Prudence is the one left to apologize and ask Mac to come back to the hospital.
In the midst of this, Foyle rears his head for the first time since the series premiere, sending Fisher a letter, promising the truth about Janey's murder. Dr. Mac insists he's just toying with her. But Butler says his late wife would tel her to persevere. Fisher goes to visit but walks out without agreeing to anything. As the episode ends, Robinson admits he knows about Foyle. Fisher asks him to tell her not to give into Foyle's deal. He won't, but hands her the letter, telling her she knows what to do. She throws it in the fire.