After skipping the cold open for the Season 3 premiere, Miss Fisher's Murder Mysteries returns to the tradition, as a young lady runs to the RAAF gates. In the darkness, a man catches up to her, and she dies. Luckily, Miss Fisher is on base the next morning, Dot in tow. She's come to visit Capt. Lyle Compton (Rodger Corser). He's an old boyfriend and head of the base, prepped to impress her with men training for Saturday's airshow, and a speedy motorbike. As they go for a ride, Dot gets a tour from Sgt. George Greaves (Huw Higginson).
Meanwhile, Jack and Collins have been called to deal with the body outside. They are on their way in to question who might have dropped some RAAF keys near it.
Jack: If we could hurry up with this break-in?
Phryne: I'm not breaking anything. I'm merely taking a circuitous route.
Phryne's here on a different case. The military suspects the union mechanics on the base of sabotage. There's a crashed plane midfield, and the pilot who walked away from the wreck, James Manning, disappeared after insisting it was due to tampering. Squadron Leader Willis Jones (Tom Hobbs), who reported Manning missing, brings his file for Fisher. But when Jack walks in, she is shocked when Compton tells him no one authorized left the base in the last 24 hours, when Jones just told her Manning disappeared yesterday afternoon.
Irritated at being stonewalled, Jack marches out, and Phryne chases after him. When he shows her a fragment of a letter found on the body, and the keys, she recognizes Manning's service number stamped on the latter, making their two cases one. As Jack leaves, Fisher discovers a bus ticket in the roll of bandages found near the body and has Dot work out where she'd been. Dot proves it could have been used to go to the European Club, the gathering spot for the Victorian Amalgamated Mechanics Association, who Manning blamed for the plane crash.
Bert's got a crush on the European Club's cloakroom lady, Tatiana Feoderoff (Kasia Kaczmarek), so he and Cec offer to take Miss Fisher along. Tatiana recognizes the dead woman as someone she sews for, confirms she'd been at the club the night before, and saw her whispering with the man in charge, Rupert Higgins (David Paterson), despite Higgins claiming he didn't know her. Tatiana also has the woman's coat, which has another letter fragment, with a piece of postmark. Dot heads off to track down the dressmaker, despite Collins trying to talk wedding details.
Collins brings in a cast of the nearby bootprints, which are a giant size 12. Manning's file says he was barely 5'6", so she was with someone else. Fisher and Jack head back to base to break into Manning's locker, where they find a photo of Manning and Jones. Upon meeting him, Jack realizes Jones' boots are a match for the prints. Unfortunately, Compton and Greaves get wind of their presence and force Jack to leave. But back at the station, Dot and Collins' investigations finally give the victim a name: Virginia Forbes, who disappeared six years ago from Claremont.
Compton tells Fisher Jones couldn't have been with Virginia. The reason for a private investigation is Manning and Jones were having an affair. If that went public, it would be giant scandal. As Greaves escorts Fisher around the base, he tells her Manning and Jones were an open secret. He believes Manning was preying on Jones, and it made the unionists disrespectful of their military bosses. But Jones is connected to Forbes; her cigarettes are in his desk. When Mac tests them, they turned out to have been spiked with ricin, which killed her. But Manning is also connected to Virginia. He transferred here from Claremont six years ago after a stay in the hospital where she worked.
With Fisher off with Compton, Jack and Collins break into the base, looking for where Jones and Forbes might have met. They find Manning's clothes and a couple of odd-shaped pillows before getting caught. As Greaves has soldiers draw guns on Jack and Collins, Fisher runs out, wearing nothing but Compton's coat, and throws herself in front of them. Compton gives in and lets them go. Back at home, Fisher and Dot finally place the "love letter" contents as a DaVinci quote about flight, causing Dot to wonder if Virginia was an aviator. Fisher has a brain wave: "Manning" was Virginia.
Jones confirms she's right. Ginny, as he calls her, took the identity and posting of the real Manning, who was committed. Virginia also believed she had found who was behind the sabotage, using Tatiana as her spy. He admits to being there when she died but insisted he had no idea what was happening, only that she stopped being able to breathe. He did take all her identifiers, including her cigarettes, not realizing they were poisoned. Since Tatiana has quit the club and is now with Bert, Fisher questions her and discovers she's not she says she is. Her real name is Tatiana Krasnaya, daughter of Geranl Anton Krasny, part of the White Army who fought for the royal family.
Virgina promised to fly her somewhere safe if she spied on Higgins, and Tatiana found proof of a plot to blow up the airshow. Tatiana was going to pass the papers to Virginia that afternoon, and Jones is now planning to show up in her stead. Given the opportunity to take out Higgins and stop a terrorist plot, Jack and Phryne set it up as a stakeout. As Fisher preps, Dot says she's sorry she doesn't love flying. Greaves made it sound so romantic, despite being stuck on the ground for the whole war at CWS. Butler notes this was where chemical weapons were tested — like ricin.
As Bert pulls up with Fisher dressed as Tatiana, Higgins sneaks through the grass with a gun as Jones lands his plane. A giant shootout occurs. Higgins begins to escape, so Fisher jumps in the plane, chasing him down by air while Jack jumps on the speedy motorbike to chase him down on land for the arrest. But while Higgins was behind the plans to blow up the airshow, he didn't kill Virginia. As Fisher realizes, that was Greaves. Ironically, he didn't even know Manning wasn't a man. He thought Manning was a mole for the reds, and gay, and that was enough.