The penultimate episode of Miss Fisher's Murder Mysteries' first season opens in Aunt Prudence's backyard, where her maid, Marigold, is floating dead in the pool. This will never do, as she's two days from throwing an engagement party for her older son Guy Stanley (Felix Williamson), Phryne's dear cousin, who was planning on making it a fancy dress costume ball. Not to worry - Miss Fisher is on the case. She beats Robinson and Collins to the scene, but barely. Jack and Phryne agree she was strangled, as Fisher finds a discarded toffee apple nearby.
Miss Fisher: Put another pin in your hat, Dot. You're in for a very fast ride.
Prudence won't admit it, but her younger son, Arthur (John Lloyd Fillingham), who is intellectually disabled, is the one who found the maid. Marigold's father, Herbert Brown (Ken Radley), Prudence's stableman, says his daughter was only 16, but she no longer lived in their cottage, staying up in the servant quarters. He and Guy confirm each other's alibis, as they saw each other in the stables around the time of death, but Phryne says Brown's claim Guy wanted horses saddled up is nonsense. (A horse kicked him as a child.) Guy's fiancée, Isabella (Kate Jenkinson), saw nothing and doesn't care. Meanwhile, the housekeeper, Mrs. Truebody, is missing, having left at the crack of dawn, quitting her position.
When Isabella calls all of Australia a backwater for Prudence canceling her engagement party, Phryne offers to make it happen, with Butler as chef and Cec and Bert as footmen. It gives Fisher eyes all over the household, though she barely needs them, as Isabella is wont to loudly announce things like Marigold's father was an abusive drunk. The neighbor confirms, "Herbert was quick to use a whip, not just on horses." A search of the stables turn up empty liquor bottles, plus a costly one he claims was a present from Guy. Brown does admit he took a whip to his daughter, out of fear she'd get knocked up.
Phryne brings Arthur his costume, only to discover he's obsessing on Marigold. In his disjointed comments, she realizes he found the body, and the toffee apple by the pool was his. He saw something too because he keeps asking if "The Woodcutter" hurt Marigold. Arthur leads Prudence and Phryne to the arboretum, where he saw Marigold and The Woodcutter, who gave him the toffee apple to go away. Arthur keeps insisting The Woodcutter is the same man who hurt Janey. Prudence insists he's merely traumatized by losing Phryne's sister Janey all those years ago.
But knowing Foyle recently attempted escape, Phryne freaks, especially since a toffee apple was a clue after Janey disappeared. Jack insists Foyle is dead, begging her to calm down. He even agrees to come to the fancy dress ball if it helps. But before that, Jack has business at the City Courthouse (the one that does divorces), a thing he'd like Miss Fisher not to know about. While he's out, Butler's downstairs gossip finds Mrs. Truebody (Andrea Swifte), the housekeeper, who insists she can only give evidence to a woman. Her scandalous story? She quit because she walked in on Guy and Isabella having a threesome.
She won't say with who, but the very expensive french undies in Marigold's things are answer enough. Isabella and Guy admit they were having fun, but things got ugly when Marigold tried to force them to take her back to London when they left. Her father caught her threatening to expose Guy, and dragged her off. (The bottle Guy was to stop Brown from beating her within an inch of her life.) They believe Herbert killed the girl anyway, Guy's "being at the stables" was a lie Brown blackmailed him into.
Phryne believes Jack about Foyle, but Dot decides she'd rather trust but verify. She fairly bullies Hugh into helping her track down Foyle's mother, and what Collins turns up isn't good. Gertrude Clarke, Foyle's abusive foster mother, has been listed as missing for two weeks. The Crematorium Manager says she visited the deceased before the cremation but never came back out. An inspection of the ashes turns up Clarke's melted glass eye in the remains: Foyle is alive. Meanwhile, just before the fancy dress ball begins, Butler eats a few hash-laced fudge pieces and has to be sent home to sleep it off. Jane, dressed as Red Riding Hood, heads down to the party as someone watches from the shadows.
Arthur runs back to his room and hides under the covers when Phryne comes to check on him, insisting he's not going to the party. But when she leaves, he hops right back out, revealing himself to be wearing wolf's clothing. When Jack arrives, Phryne leads him upstairs to her room, so that he can change into a Marc Anthony costume to her Cleopatra. But when she picks up a note left on her table, she discovers it has the other ribbon from Janey's hair from the day she disappeared. They race to Arthur's room and realize he's gone too.
Downstairs, Cec and Bert are supposed to be watching Jane but get distracted, and she slips off when the wolf beckons in the chaos of the party's treasure hunt. Jack and Phryne find her, as the wolf runs off, but it's not Arthur in the costume. When Guy finds his brother, Arthur claims he gave the outfit to The Woodcutter in exchange for a toffee apple. Dot and Collins arrive with the news about Foyle. Meanwhile, as Cec, Bert, and Jane head out in the car, Foyle approaches them, asking for a ride. Cec and Bert drop off Jane first, then Foyle. He sneaks back to Fisher's house and appears at the door, asking Jane to use the phone. He cuts the line before making himself at home in the kitchen.
Luckily, Butler wakes up and spooks Foyle, who leaves in a hurry. When Jack and Phryne arrive home, they discover a peaceful scene of Butler and Jane having a late supper, and Foyle gone. But for how long? There's only one episode left in the season to find out.