Is this Miss Fisher's Murder Mysteries or Victorian London? The cold open features cobblestone allies and beggar children running for their lives — all save one, who is already dead.
Cut to Dr. Mac, attending a presentation by Dr. Harcourt (Dan Spielman) concerning those still suffering the scars of the Great War. He's using patient Archie Woods (James O'Connell) as his example of why Lady Grenville and the wealthy matrons of the Ladies Auxiliary should continue their charitable giving. But the presentation is interrupted when a nurse bursts in, having found a dead child.
Fisher: I'm sure he's not the first man who wanted to wring my neck.
As Miss Fisher returns home after an all-nighter, there's a knock just behind her. A young teen named Paddy (Jarin Towney) bursts in, hoping to hire her to find his brother, Ned (Ben Keller), who didn't come home last night. "Home" is the Collingwood slums where Fisher grew up. Paddy and Ned are orphans who live on what they can beg, borrow, or steal. (Paddy's "payment" is a silver candlestick.) When Mac calls both Jack and Fisher to alert them to her new case, Fisher realizes it's likely Ned. But though Paddy recognizes the body, it's not his brother, but another member of the BottleTop Boys gang, Badger. He disappeared the day before Ned did.
Harcourt's autopsy shows he died of a stab wound, though Fisher notes the kid is covered in cuts. He also has a feather in his hair, which Butler identifies as duck. Paddy clams up in front of Jack, but tells Fisher the bullseye lollies in Badger's pocket come from "The Monster Man." As Jack and Fisher question the staff, they realize Woods has a bottle of the same treats, and his facial prosthetic is exactly what young boys would think of as a "monster." Woods admits Badger did delivery jobs, which he paid in candy, but is genuinely shaken to learn the kid's been killed. Harcourt insists Woods wouldn't be capable of killing; in fact, he's only employed as a way for then hospital to keep tabs on him.
Down in Collingwood, Paddy, Fisher, and Dot run into Mary Maddison (Diana Glenn), who drives the health van. She identifies Badger as Barnaby Parkins and is distressed to discover Neddy's gone missing. She asks Fisher if they're also looking for Jimmy, who hasn't checked in for over a week, as Woods stops by to do a drop-off and take stuff back to the hospital. Mary then leads Fisher to the BottleTop Boys hangout and Col Richards (Matthew Testro), the gang leader. When no-one talks, she has Bert drag Col down to the station. Col claims innocence. Badger was out of the gang anyway, working for someone new.
Fisher decides to send Paddy in undercover to volunteer to do Woods' odd jobs to see if a lead turns out. She was expecting petty theft, but it turns out, Woods is stealing morphine from the hospital, and Badger was his drug runner. When confronted, he goes nuts, grabbing a sword and nearly running Fisher through with it. But she notes how easily Mary soothes Woods, and wonders if the delivery she saw him make to the van in Collingwood was a drug deal.
Hugh is gunning for a junior detective position; he needs the money because his mother threw him out when she learned he was converting. Jack is happy to help, giving Collins extra shifts and letting him get more involved in the case. That how he finds himself down in Collingwood that night hunting for Ned. It's a good thing, too, since Paddy snuck out, and Collins stumbles across the kid, just as they both find Jimmy's body. Collins also finds a necklace in Paddy's pockets, but he insists he found it tucked in the secret communication spot with Ned, proving his brother's alive. Fisher recognizes it as matching the earrings Mary was wearing.
Fisher discovers the "packages" marked for Woods are filled with jewelry and gold watches. But they're not payment for drugs. Mary says she buys them off the boys, and Woods pawns them for her. It's all to make money to support the van, she insists, and if the items were stolen, she didn't know. Col would have told the boys to do it. But Richards insists Badger's petty thievery wasn't under his orders. Jack releases Woods, but unfortunately, he turns up dead the next morning, having overdosed on morphine. His latest skin graft was not going well, and his eye socket was infected.
Collins finds the murder weapon, but the fingerprints aren't Richards, they're Ned's. The boy confesses Richards gave him the knife to scare Badger back in line. But they fought, and he accidentally stabbed Badger and ran. Jack has to arrest the kid, much to Paddy's distress. Fisher follows Jack back to the station, insisting that this doesn't add up. But Collins has been tracking Fisher's suspicions of Mary and discovered "Mary Maddison" doesn't exist. She's Velma Mary Ferguson, wanted for the murder of her husband. Mary says it was self-defense, and Dr. Harcourt knows the truth. He removed her identifying birthmarks, in exchange for her running the van.
Fisher realizes Ned confessed to the stabbing, but someone else moved the body. She has Mac take another look at Badger, and they realize the stab wound wasn't fatal; Harcourt lied on the autopsy. He's been using the kids for their skin, to get material for his graft experiments on Woods. He paid them to let him cut off sections of their skin and then didn't check they took care of themselves, which is why Jimmy died of infections to his wounds. Badger threatened to tell when Hardcourt wouldn't stitch up the stab wound from Ned, so Harcourt suffocated him. Harcourt insists he is working on revolutionary science. One day donor skin, and even organs, could be used to save lives. Who cares if some orphans die for that?
The case may be solved, but Collins loses out on the promotion. Desperate for money, he quits the police force after a blowout fight with Dot. Jack refuses to accept his resignation. Aware Collins is terrified of telling Dot what's going on; he steps in to let her know. Meanwhile, Butler hires Paddy and Ned to do odd jobs for the household, which will hopefully pay enough to keep them off the streets from here on out.