Miss Fisher's Murder Mysteries was nearly canceled after its second season; it was a fan campaign on social media that earned it a reprieve.
As we round into this last (and much shorter) season, it begins at Mackenzie's Cavalcade of Mysteries. But this is no cold open. Dot, Bert, and Cec are here. And it's a heck of a show as the trick guillotine fails to trick, and poor Miss Pearl loses her head. Meanwhile, Phryne's dinner with Jack has canceled due to the arrival of the Baron of Richmond, Henry Fisher (Pip Miller), her father, who happens to be their patron.
Miss Fisher: Men: Can't live without them, can't hit them with an ax.
Jack and Phryne meet at the circus, where she explains trick guillotines, and proves this was no accident. Backstage, they meet Pearl's fiance, owner Merve MacKenzie (Grant Piro), and his assistant, Eva Callahan (Kate Mulvany), whose father used to own the show. Eva admits the guillotine was kept in the props storage, so anyone who went in there could have tampered with it, including props master Sid Forrest (André de Vanny) and the contortionist Sarah Norden (Eloise Mignon). Sarah was Pearl's BFF, and Fisher spots her gathering Pearl's necklace out of the pool of blood.
Sid says he checked the guillotine but admits he was concentrating on the tank he's been readying for the reintroduction of the Mermaid Act. Fisher asks Sid about his ambitions and gets out of him that Eva's been quietly training the tech all her father's old tricks. Speaking of her father, Henry shows up to take her to dinner. Instead, she takes him home, demanding an explanation, where he goes on about the reintroduction of the Mermaid tank trick, and how it will put him right financially again if someone would float him for a spell. She agrees and then kicks him out to a hotel.
Dr. Mac is now in charge of autopsies, though unfortunately, there's not much for her to do this week. Also, Dottie's notes contradict Jack's written statements that Eva set up the guillotine, so she comes in for questioning. Eva admits she lied but insists she taught Sid so he could take over the job of checking props so she could make money doing fortune-telling, not because she was grooming him. She insinuates Sid didn't like Pearl, which he admits is true when Jack and Fisher track him down. He had the opportunity to tamper with the guillotine, and evidence of that tampering is hiding in the paint for the mermaid tank restoration.
Aunt Prudence shows up to complain someone is trying to charge the royal suite at Melbourne's fanciest hotel to her account, which she declined. Of course, that means Henry's back, much to both women's annoyance. Fisher goes back to the circus where Dot has spotted Sid and Sarah conspiring. Henry shows up there too, as Mackenzie works with Pearl's new replacement, Mavis (Irene Chen). He hands over a check from the money Phryne floated him, but his offering only makes Eva realize the cashbox has been robbed. Fisher realizes that Sarah is not only wearing her former BFF's pearls but has the missing cash in her sweater. In response, Sarah runs.
Sarah gives Fisher the slip, but not before making her late for her dinner with Jack, who gets drunk while waiting and then accidentally knocked out by Henry in the middle of trying to confess his feelings. Jack wakes the next morning and finally learns Fisher's father is the circus' financier. Meanwhile, Cec and Bert realize Sarah disappeared by contorting herself into a trunk and deliver it to the Fisher household. Under questioning, Sarah reveals she and Pearl were lovers, and Pearl only agreed to marry Mackenzie to get the Mermaid act. She insists the money was for Pearl's headstone, but that's shown to be a lie. That being said, both Sarah and Sid are petty thieves, not hardened murderers.
Fisher also turns up pictures from Mackenzie's first wedding. The bride looks like Eva but is actually her late twin sister Millie. Mackenzie claims Millie drowned doing the Mermaid routine, giving himself up for manslaughter and negligence. But Eva drops a letter addressed from Millie, which Dot finds. They track down Millie, who admits she staged her death to get out of the relationship — Mackenzie was controlling and abusive. But when a search of Millie's things turns up paregoric, an opium-based drug, Dot decides to return to the house under the guise of giving her a love letter Mackenzie wrote when he learned she was alive.
Every suspect in the case is trying to stop the Mermaid trick from coming back, so Phryne attempts to flush out the killer by volunteering to do it herself. The act seems to be going well, with mermaid dancers and Fisher making a daring entrance. Meanwhile, Dot discovers no one home at Millie's, so she breaks in and finds the house all closed up, and a ton of mail addressed to someone else. Collins and Jack arrive to help Dot and see more of the paregoric, which Collins notes his cousin uses for asthma.
Dot realizes that the paregoric must be Eva's, as she admitted to having trouble breathing whenever she got nervous. (That's why she couldn't take over the Mermaid trick from her sister.) The three realize in horror that Millie isn't alive after all. The woman pretending to be "Millie" was Eva. She killed her sister out of jealousy, feeling cut out of the limelight. Back on stage, Fisher reaches the same conclusion just as she's lowered into the tank. Millie starts gasping for air and pulls out her paregoric. When Fisher puts two and two together, Eva pulls the release tool out of Fisher's turban and shoves her in the tank to die.
As Fisher struggles in the tank to free her chains, Jack, Collins, and Dot arrive, just as Henry realizes Phryne is trapped and panics. But as Jack runs up to the tank with an ax, the curtain rises to reveal Phryne freed herself. She had additional tools tucked away inside her costume just in case. If only she could find a trick to make Henry disappear. But unfortunately, Prudence has uncovered he's squandered his entire fortune. It's going to take quite a bit of money to make that magic happen.