It's a rare Miss Fisher's Murder Mysteries that doesn't start with a cold open, but this week, we begin with Hugh and Dot on a date. They're fishing and talking about honeymoon plans when Dot accidentally catches the body of a teenage girl on her line.
For all Hugh tries to shield her, Dot helpfully identifies the Magdalene Laundry mark on the dead body's clothes from the Convent of the Sister of the Holy Miracle. Jack and Phryne head to see Mother Aloysius (Joan Sydney), who identifies the girl as Bernadette. Perpetua (Shayne Francis), who places these orphaned girls, had gotten her a job at the Federal Hotel, but the girl had just quit and came back. Sister Dominica (Sally-Anne Upton) said she'd disappeared before morning.
Jack: Don't be remorseful; it only confuses me.
Jack and Phryne are taken to Bernadette's bedroom through the laundry, a horrific sweatshop. Worse, the "bedroom" is actually 'The Penitence Room." Perpetua says they put her here because it was after midnight. But she also states Bernadette returned because she "decided being a kitchen maid was beneath her" in a highly disapproving manner. The room is currently occupied, by the way, by Mary (Alice Cavanagh), pregnant and put out that the convent refuses to pay the workers for their labor. Fisher is horrified, especially once Perpetua and Dominica present her with sheets clumsily tied to look like Bernadette used them to escape, but with knots that would never hold her.
Fisher's attitude and Jack's defense of her cause the Bishop to telephones a complaint to Sanderson. Obliged to keep the church happy, he tells Jack the case is reassigned to DI O'Shaughnessy, a total incompetent, but at least he's Catholic. Sanderson has just been promoted to Chief Commissioner and says he does not want to make waves. Back at Fisher's, Aunt Prudence is defending the church's labor practices, casually noting the better-behaved land in her Gratitude Girls program. But two have recently disappeared, including Joan (Fantine Banulski), who was assigned to her household.
Despite Fisher's determination to find another launderer, Cec and Bert turn up with laundry for Sister Dominica. Once dropped off, Phryne pops out to investigate. Mary catches her immediately and agrees to help. She was in the Penitence Room when Bernadette arrived, hysterical, begging not to let "him" get her and going on about a car and Joan, who was her BFF. She also scratched a message on the Penitence room wall using Joan's glasses: 112 De Vere. But sadly, Fisher and Mary are caught, just as Sanderson promises Perpetua, he's reassigned the case.
Sanderson drags Fisher to the station demanding Jack control her, just as Sidney Fletcher and Rosie walk in. As members of the Gratitude Board, they want to help. Rosie also wants to know why Jack's been reassigned, and her eyebrows go up when Sanderson says if Fisher is caught again, Jack will be suspended. Since he's not officially on the case, Jack takes the autopsy to Fisher's to brainstorm how saltwater and red algae got in Bernadette's lungs. Bert suggests a ship dumping water ballast at the docks. It turns out a boat just came in from the Red Sea; the Pandarus captained by Wilhelm De Vere (Paul Moder), where there's a fancy car with plate number 112. Fisher sneaks on board, finding Bernadette's hair tie in De Vere's cabin.
Jack and Phryne's inquiries do not go unnoticed, which is enough to speed up the ship's departure. Fisher's ready to go stop the boat herself, but Jack begs her not to. Instead, he goes to Sanderson, who agrees it's enough for a warrant, declaring he'll head up the raid. If Jack interferes, he'll be dismissed. Thankfully, Fisher is not staying put, and Butler even arms Cec and Bert, a first for the series. Also, Fisher's not done meddling with the convent either and helped Mary break out. She's asked Prudence to take the girl on as a project. None too soon, it turns out, because the baby is on its way. With everyone out, Prudence and Butler oversee the birth.
On the boat, Fisher finds a terrified Joan, who reveals there are other girls bound and gagged onboard. It's a sex trafficking ring, stealing them away. Unfortunately, just as Fisher finds the girls, Sidney finds her. He's been using the Gratitude Board to select girls, his most valuable trading commodity. Soon enough, everyone is bound and gagged, with the ship ready for departure.
When Collins turns up Sidney's company owns the 112 car, Jack realizes Sanderson must know the truth. The two drive down to the docks where there is no raid; Sanderson is protecting the ship until it can set sail. Jack boards it anyway and when he finds Miss Fisher's lockpick, he realizes she's somewhere on the boat. Phryne escapes, alerting Jack just as the shots start firing. Fletcher tries to escape, but Fisher catches up to him. He pulls a gun on her and Jack fires, knocking him backward into the sea. Dot walks the girls back to the car, while Fisher walks Sanderson off the ship at gunpoint.
As this is the final episode before the Christmas special, it all ties back to the Brothel Scandal from the premiere. Fletcher had possession of the nicked items from Lyon's "second secret registry" of high-profile clients, including Commissioner Hall's badge to coerce them into turning a blind eye. Since he couldn't blackmail the upright Sanderson, he used the promise of securing the promotion to Chief Commissioner to bribe him. Sanderson claims they can't prove this, but Fisher counters Joan wasn't abducted for sale. She was taken because she overheard Fletcher bribing Sanderson at Prudence's. When he sneers that no one will believe her, Fisher reminds him Prudence is very fond of Joan and will make sure her word is taken seriously.
Back at the station, Perpetua admits she sent the girls to Fletcher, insisting to a horrified Mother Aloysius this was all for the good of the church's coffers, as the fallen girls were whores anyway. Rosie is also horrified, having learned her fiance trafficked little girls, and her father knew it, refusing to listen to Sanderson's pleas of innocence, collapsing against Jack sobbing. Fisher heads out, not willing to disturb them, only to find a baby in her parlor and Dot and Prudence cooing over it. But by the end of the episode, Jack has come by to say goodnight. Even Rosie may not be enough to stop this ship from sailing.