Though fans hoped a Miss Fisher's Murder Mysteries Season 4 could occur, this final episode feels like the writers knew they were wrapping it up, starting with the only "last time on Miss Fisher" precap of the entire series.
Then it drops into a cold open of Dr. Quentin Tode (David James) receiving the Wren Science Prize, as our Mysterious Murderer from "Death at the Grand" awakens from his coma, attacks the nurse and escapes. As he arrives at the ceremony, disguises as a policeman, Father O'Leary makes a scene, proclaiming religious doctrine over science. Osman Efendi (Matt Zeremes) also has uncomfortable questions about Tode's research but is dismissed just as quickly as the priest.
Phryne: It all looks very still to me
Jack: That's because you're not a telescope.
Jack heads to Fisher's, letting her know Mysterious Murderer escaped. Henry's ship sails the next day, so she only has to keep him safe for 24 hours. Jack says the only lead they have is an article about the Wren Insitute of Science found in the Murderer's room. Bulter walks in, and says Collins has telephoned, as Tode's body has been reported by the gatekeeper (Matthew Quartermaine) at (where else?) the Wren Insistute of Science. Fisher leaves Henry under Butler's watchful eye, as she and Jack head to the equatorial room where Tode was found, his skull cracked. In his pockets, there's a lady's wristwatch engraved "Remember," but nothing to tie him to the Mysterious Murder or Henry, other than being English.
The gatekeeper believes Tode was killed by aliens, having seen a figure floating in the darkness with a glowing ring around his neck. (With science like this, no wonder O'Leary is miffed.) The priest seems to have taken to leaving psalms outside Tode's door, confusing the situation further. Dot confirms the note is in the priest's hand, and Fisher realizes the alien "floating" was the robe over grass, and the white clerical collar was what "glowed." Dot looks into the watch as Hugh asks O'Leary to come to the station for questioning. O'Leary admits he took a swing at Tode after the ceremony for saying there was no God but did not kill him.
As for Efendi, when Jack and Fisher question him, he insists he doesn't recognize the Murderer, but Tode claimed to feel ill since his arrival. The head of the Insitute, Logan Wren (Tamblyn Lord), notes the £50k prizemoney Tode received was untouched, and he confirms Tode was feeling unwell after the ceremony. Also, with his passing, the prize money will go to the first runner up, Efendi. Mac determines Tode was poisoned, and there's a clue in O'Leary's "glowing," as Collins discovers everywhere Tode put his hands on the priest shine bright blue in the dark. It's not just a phosphorescent, but a radioactive one, polonium.
As they search Tode's things, Fisher discovers that Tode had Efendi's research papers and used them to fuel his work. Also, Collins finds one of Efendi's teapots hidden in the bushes, suggesting he might be the one doing the poisoning, especially since he signed for the polonium delivery to the facility, which is now missing. Efendi insists Tode stole his research. But when confronted with the teapot, he insists he didn't poison anyone. He gave it to our Mysterious Murderer. He calls the man "Denizci," Turkish for "sailor." They met during the War, Denizci saved Efendi's life when their ships were sunk in a sea battle in 1915. He says Denizci was mad from shellshock, but wouldn't have been after Tode.
Dot returns from the jewelers, confirming the watch was bought in the UK, but engraved in Australia, only to have Fisher have her look into anything Tode might have done with polonium. Back at home, Henry receives a letter, which turns out to be a note from Denizci telling him to come out to the rear lane. With Phryne out, Henry starts patrolling the kitchen, looking for a moment to escape, and finding the invite for the Wren Institute award. He slips a downer into Bert's tea and escapes, only to be kidnapped immediately, and driven off in Bert's cab.
With Henry disappeared, Fisher goes through his things and discovers a letter from Margaret, her mother threatening to divorce him if he doesn't come home. But Henry has more in mind that Margaret. He tells Denizci they had a deal, which Denizci says Henry broke. Their argument is interrupted by Efendi, who arrives to tell Denizci to give himself up. When Denizci refuses, Efendi calls Fisher, but before he can give the address of where Henry is being held, Denizci murders him. As Fisher looks to find the address among Efendi's things, Dot arrives with the news Tode invented a short-lived "glowing paint" for Meyer Towers, who are clock and watchmakers. They made the woman's watch in Tode's possession.
Collins looks into Efendi's story and finally identifies "Denizci." The Captain of the Battleship Ocean was Eugene Fisher, Henry's cousin and heir to the Fisher title, presumed drowned in 1915. When he turned up years later, Henry paid him to go away rather than face Eugene being court marshaled for desertion. Henry's goal is to pay Eugene off again, suggesting they steal the Wren prizemoney. Dot is at the institute researching who died from Tode's paint when Eugene leads Henry in at gunpoint. Fisher runs in with Jack, and Eugene forces her to open the safe. Inside, the missing polonium sits next to the money. She distracts Eugene long enough for Henry to attack him, and Jack arrests him.
Henry admits his whole trip to Australia was to escape from Eugene. He lied about losing all his money at cards so Margaret wouldn't ask questions. Now he's missed the ship home, and ruined his marriage -- until Phryne has an idea of a faster way to England: Her plane. However, while Henry's murderous stalker has been taken in, that still doesn't answer who killed Tode, until Dot reveals the list of Meyer factory workers who died from the paint includes Wren's late wife. Wren admits he knew Tode was a fraud. The award was an excuse to invite him here, with plans for revenge. He has no regrets.
Faced with Phryne leaving for England, Collins works a miracle of his own, convincing O'Leary to marry himself and Dot in a last-minute wedding. After all, Dot couldn't get married without Miss Fisher to walk her down the aisle. Also, Jack's clumsy "romantic overture" proposal to make the stars move for Phryne may have finally happened off-camera. (There *was* a shooting star after Dot's wedding!) But he's not just going to let her leave without seeing her off. She responses with a romantic overture of her own: "Come after me, Jack."
As she gets in the plane, he watches her take off. Maybe he just will.