This week's Miss Fisher's Murder Mysteries opens at another carnival, Biggs Boxing Troupe, as a dead man falls through a poster. The next morning, Collins is over at Fisher's, getting patched up by Dot after a brawl at the boxing gym he's been running for wayward youth. Conveniently, both she and Dot are in the room to hear about the murder when Jack rings. Collins recognizes the dead man on arrival: Kevin Bradley, the head of the Woopackers gang, one of the two rivals his gym is targeting to keep off the streets. But Collins swears he didn't see any knives at last night's brawl, and Kevin wasn't there anyway.
Dr. Mac: So should I just put this brain back where I found it?
The issue turns out to be more than just one brawl. There was a melee a week prior, where an officer, Constable Fry, was stabbed and died. Collins was there, and says it would have gone nowhere had the Woolpackers' main rival, the Portsiders, not turned up. Flour bombs flew — the Woolpacker's second, Freckles Delahunty (Scott Smart), is a baker's apprentice — as Tom Derrimut (Mark Coles Smith), the leader of the Portsiders, attempted to break things up. Fry chased a gang member down an alleyway and never came back out.
Fisher wonders if Bradley was a regular boxer, but Arthur Biggs (Brett Swain), who runs the Troupe, says he's never seen him. His wife, called Mrs. Arthur (Glenda Linscott), concurs. Fisher follows her out to her laundry pile in hopes of more information. But like Collins, she suspects this was police retaliation for the death of Fry. But the Portsiders also lost a member the night Fry was stabbed, an Aboriginal teen beaten and left at the steps of the local hospital. (The Woolpackers are whites only.) Phryne takes herself down to Collins' gym, Dot in tow, to see the half-naked sights and ask a few questions.
Collins is sympathetic towards these fatherless kids. (His father died in an accident trying to break up a fight, where he was knocked back and cracked his head on the gutter.) He's working with Tom when they arrive, but within a moment of Fisher introducing herself, Freckles and his gang arrive accusing Tom of killing Kevin as Portsider revenge. Tom insists he was out, and Kevin came after him for no reason. But Collins brings him in to give a statement, where he points the finger at the Tenners.
Fisher and Dot head back to the carnival to ask questions, while Jack is also on site. He's interviewing Giorgos the Greek (Chris Asimos), who found Kevin's body. While they chat, Fisher overhears Biggs threatening Tom's mother, Cora (Rachael Maza), Tom better keep his mouth shut. Phryne also finds the murder weapon, tossed on top of a tent. But before she can get her hands on it, Giorgos pulls her down and offers her tickets to tomorrow's show. Seeing Cora getting away, she points Jack towards the murder weapon.
Fisher catches up as Cora and Tom are arguing. Tom protests that he wants out as Cora counters Freckles has taken over "demanding payment." Fisher pulls Cora over, citing Collins' concerns for Tom. Cora says Tom is a fine Bunurong man, and innocent. She's already lost her husband to the war, Welfare took her younger son as part of what will come to be known as the Stolen Generations. Fisher uses what she learned to ask Tom at Collins' gym why he's boxing for Biggs. Tom won't answer.
Dr. Mac detects signs of CRT in Kevin's brain. (Though of course, no one calls it that.) With this proof he boxed regularly, Jack goes back to Biggs, who admits Kevin and Tom boxed, but only as challengers against the Troup. Jack notes one of their Troupe is unaccounted for, the "Black Belter." Biggs says the kid was named Jimmy and insists he just disappeared one day, which upsets Mrs. Arthur. Dr. Mac notes that the Portsider body that showed up the night Fry was stabbed wasn't covered in flour, but washing powder, just like the stuff Mrs. Biggs uses.
Butler, Bert, and Cec pool their knowledge of boxing to come up with the working theory Biggs is fixing fights for betting odds, which also explains the bits of money left in Kevin's jacket. Fisher gives Bert and Cec pin money to go undercover betting. Upon arrival, Tom is being advertised at "The Black Belter." Dot notices a boy pickpocketing, who tries to give the cash to Tom as Fisher then realizes the picture of the dead "Portsider" is the same face on the "Black Belter" poster. When she and Jack go to ask Biggs about it, Mrs. Arthur is attacking her husband for redoing the banner for "painting over my Jimmy." Biggs admits Jimmy was beaten near to death by Kevin in the ring. The washing powder was to hide where the injuries came from.
The knife used on Kevin turns out to be the same one in the Fry stabbing. Jack says it's a German bayonet from the war, and Fisher notes it has a possum carved on it. Tom's father's name was Wallet, which means "possum" in Yolŋu. Collins confirms that Tom referred to his late father as "Possum Derrimut." Collins tracks Tom to the boxing ring, about to fight a Woolpacker challenger. Bert and Cec see Freckles filling the gloves with lead. Mrs. Arthur is hysterical Tom will be killed, and Fisher demands the fight be stooped. Collins finally jumps in, saying he'll finish instead, and knocks the challenger. But before they can take Tom away, the boy runs up, demanding to confess.
He's Dan Derrimut (Isaiah Lane), Cora's younger son, who ran away and came back to his family. He was at the brawl; it was him Fry chased down the alley as an easy target. Tom jumped in, and Dan picked up the knife just as the cop stumbled backward into it. His death was an accident. Kevin stumbled in after it happened and assumed Dan did it on purpose, that's why he was blackmailing Cora. But the Derrimuts didn't kill Kevin. When Mrs. Arthur saw Cora pull the knife on him, she jumped it and did it herself, as revenge for Jimmy.
Fisher initially tries to celebrate by bringign home the Greek but he's too into himself to sleep with her. When Jack shows up to take her on the roller coaster, she happily walks out for a date.