This week's Miss Fisher's Murder Mysteries cold open begins with Miss Gerty Haynes roaring off in her car. Someone quietly follows, but no one goes far. One of the wheels pops off, and Gerty loses control of the vehicle, driving headlong into a tree. The next morning, Miss Fisher is playing billiards with Dr. Mac and Ailsa Wilton (Rachael Blake). Miss Fisher has just founded The Adventuress' Club, which has made a substantial donation to this Saturday's ladies' racecar team. Unfortunately, that's when Ailsa's daughter Millie (Nikita Leigh-Pritchard) runs in to announce there's been an accident.
Jack: Even a Celtic queen has the odd accident.
Jack Robinson arrives on the scene, Collins having called him in, despite vehicular accidents not being his thing. Fisher is already there; she demanded Collins call him. (No wonder Jack thought it was going to be Phryne's body in the car.) Fisher insists this is murder. Jack, still shaken, fights her on it, proclaiming it an accident. He also insists on driving separately to the racetrack to inform Gerty's brother Claude (Rohan Browne) of his sister's passing. Claude, it seems, works for a competing team.
Claude's sponsor, Lachlan Pepper (David Roberts), a would-be MP, agrees with Jack, insisting women are incapable of driving. Fisher declares the ladies' team is not pulling out. And if they win, Pepper will have to allow women to become full members of the Victoria Automobile Association. Pepper, in shock, agrees. He's not the only sexist idiot about cars, sadly. Collins manages to put his foot in it saying cars are far harder to work than sewing machines. Dot's face says everything you need to know about that. Millie sthinks she can fix the vehicle; all Miss Fisher needs is to find a driver.
Ailsa says the two of them had a spat before Gerty left. It was over a car motor they'd ordered, which Gerty forgot to pick up, and by the time she got to the docks, it was gone. The VAA doorman won't let Fisher in; women must be signed in by a man. But he tells her about the other car, which had a taillight out. He also points to where Gerty was parked. The wheel nut is lying in the gutter, obviously removed by someone. Fisher also questions Anthony Rose (Shaun Goss), Haynes' "latest toy, " who claims not to have been at the VAA lounge that night. As they talk, Rose reveals Claude's "secret weapon," for this Saturday, the missing motor his sister had ordered for her car. Miss Fisher response the ladies have a secret weapon too: She's driving.
Pepper freaks out when he finds out Fisher is taking Gerty's place and insists her speeding offenses disqualify her. Fisher responds that the stolen motor as proof Pepper was sabotaging the women's team, which wouldn't look good for his political career, but no luck.Papper won't let her race. She turns to Jack to protest, but he's still upset about thinking she was dead. He now agrees Gerty was murdered -- strangled in fact. But he's not letting her sit in on Claude's interview.
Claude insists he didn't steal the motor; Gerty knew he had it. They had a deal for her to throw the race. Father had cut Gerty off, insisting she marry and settle down. Claude had made Gerty give him the motor because he didn't trust her to throw it. No one knew, not even Ailsa, who would have gone into a rage had she found out. Jack heads over to Wilton's garage to take Ailsa in for questioning, but Millie stops him. She went to the VAA that night and removed the wheel nut after she heard her mother and Gerty fighting. Jack goes off at how foolish her actions were. But Millie didn't kill Gerty. And she can confirm both Pepper and Rose's cars were at the VAA lounge that night.
Fisher has found a new driver: Dot, even though she's no idea how to drive. Mr. Bulter forges her a Russian driver's license, making her Miss Valentina Ranemalova, ladies racing champion. As Pepper tries to comprehend this, Fisher grabs the chance to check out Rose's car, which is missing a taillight and has Gerty's purse tucked away inside it with a note to from someone asking to meet. Rose says he did drive up to the VAA that night, but never went in, taking off with a female fan to a hotel. Also, it's not his handwriting. He insists Gerty had left him; she found "someone better" who was taking her overseas.
Fisher has Claude take "Valentina" for a walk so Cec and Bert can steal the motor back for the ladies' team. But it's already been removed. There's a note for her from Pepper next to the car. But it turns out Pepper thinks she stole it. It's also the same handwriting as the note found in Gerty's bag, which drives him to accuse Fisher of being behind an ongoing scheme to blackmail him over a fling he and Gerty had years ago, and a pregnancy that followed. Jack discovers Pepper signed Gerty in, and witnesses said they had a row in a private booth. Dr. Mac confirms, after a look at the body, Gerty carried the baby to term. An illegitimate child coming to light is something no man running for office would want public.
A bit of research puts the pregnancy at 16 years ago. Funny, Millie is just 16 herself... As Gerty's BFF, Ailsa would have known why her friend was off in London for six months. Confronted, Ailsa tells Fisher the truth. But Millie has her daughter from the day she was born. Gerty chose racing. Their fight was because Gerty thought producing a granddaughter would change her father's mind, using the child to get the money. Ailsa followed Gerty to the VAA, wanting to convince her not to take Millie. She wound up following her and, in a fit of passion, strangled her after the crash. The reason she grabbed the purse was the other secret document Gerty was carrying: Millie's real birth certificate.
Jack barges up with the event about to start, but Fisher lets Ailsa escape to watch the race and say goodbye to Millie. But she doesn't get to switch places with Dot in time, so Millie trades out to drive for the women's team instead. Just as the race is supposed to start, Claude runs in. Rose stole the motor. Desperate to win the race, Fisher jumps in Claude's car, and blocks Rose partway down the racetrack. The ladies are victorious.
Pepper is horrified, demanding an arrest since Fisher and Dot made a fool of him. Fisher counters in front of Millie he could take this opportunity to admit his long-lost daughter as the first female member of the VAA. It's a triumph all around, but Jack still isn't over the idea he could lose Phryne. The episode ends with him attempting to break up their partnership.