The problem Australian TV has with doing Christmas specials is the holiday lands in midsummer where they are, whereas in most of the Western world to which they export, Christmas brings images of snow. Miss Fisher's Murder Mysteries solves that by having Aunt Prudence organize "Christmas In July" and sending the show to the Australian Alps and "The Snowies."
The cold open features a man getting electrocuted while trying to get the Christmas lights to work, just ahead of Fisher's arrival. She and Prudence are staying with Nicholas Mortimer (Simon Burke), the family's business partner in mining, who greets them in shock over having found the body that morning.
Collins: Miss Fisher's gone on holiday again, sir.
Jack: Oh. Anyone dead yet?
Collins: Only one so far, sir.
The deceased was Len Fowler, former foreman of the Stanely Mines. His wife, Birdie (Alicia Gardiner), is upstairs grieving, while Chester Groves (George Shevtsov), the mines' former manager, frets at not being able to get a doctor. Luckily, Fisher brought along Dr. Mac. The holes in the slippers and burnt finger are enough to prove this was no "heart attack." Fisher checks the living room Christmas tree, and finds the pear decor lights and the removed bulb lying behind the tree before Quentin Lynch (Greg Saunders), Mr. Mortimer's secretary, whisks them away.
Up in their rooms, Fisher and Dot find welcome cards correlating to "12 Days of Christmas," Fisher has "Nine" (ladies dancing), Dot has Seven (swans a-swimming). Fisher goes to explore and finds Nicholas reading Wordsworth, and spots a photo of the miners he removed from the wall. He explains it upset his wife, Vera (Sylvie de Crespigny). These were the men who perished in the infamous Stanley Mines 1919 Christmas Eve accident. Among those who died: Quentin's uncle, Ernie Heppenstall, and Mitch, Vera's first husband and father to Isobel (Emily Milledge), Nicholas' angry teen stepdaughter.
A storm arrives along with the after-dinner drinks, and Chester notes the road will probably close. As Fisher helps Birdie hang the stocks, she discovers one for "Laurie," the mining accident's youngest victim. Prudence, upset by the subject, reveals her other purpose in coming is to sign away the mine to be sold. Vera is horrified at the idea, but after Isobel blames her mother for remarrying, she heads upstairs in distress. She, too, is murdered by a statue of lovers attached to trickline triggered by her favorite Wordsworth volume.
Jane arrives home from boarding school early, but Bert's cab is unable to go up mountain passes, so Butler declares they'll have their own Christmas in July. Meanwhile, Dot has called Hugh. He and Jack make it up the road just before it closes, arriving as Vera's body is found. With the gang all here, Quentin immediately accuses Nicholas of killing Vera. In his grief, Quentin reveals he's been having an affair with her, confirmed by a note in the Wordsworth volume, which shows Quentin had been pushing Vera to leave Nicholas for him.
The next morning, Nicholas is found shot in the face in the hen house. Quentin has disappeared along with Miss Fisher's skis. The phone's been disconnected too, but with Quentin gone, Jack assumes everyone is currently safe. Birdie tries to get the Christmas cheer going with "12 Days of Christmas," but it causes Prudence to meltdown. It turns out that was the carol she thought she heard the miners singing as they waited to die after the cave-in. (Shout out to John Noble cameoing as Prudence's late husband Edward in the flashback!) Meanwhile, Isobel's acting out draws Fisher and Jack's attention. They discover money in her room and the "Five" (gold rings) card.
Jack's supposition the group is safe is wrong, as Prudence is pushed into a frozen fountain after she hears calling birds outside. As Dot pulls out Prudence's powders from her purse, the "Four" card falls out. Fisher realizes these are the order in which people are being offed. Assuming Quentin is still here, Jack has the house locked down, only to realize Mac has disappeared. They find her outside, nearly frozen. She's been attacked out of sequence (she's six), But her location explains why: She saw the mine shaft was recently opened. Birdie attempts to shut down Fisher's questions if Len could have opened the mine until Fisher hits her with a query about Isobel's money.
Birdie reveals that Nicholas recently paid Len to find a new way to the gold they'd been aiming for when the collapse happened ten years ago. Along with the gold, they'd also found the bodies, with a note proving the miners had been alive after the collapse. The child, Laurie, had been sent up to tell Edward, Nicholas, and Chester, but he supposedly died before relaying the message. Later that evening, a second collapse happened, which Birdie now believes was an explosion, engineered to kill the men. She also admits they were going to tell Prudence everything if Nicholas didn't cut them into the deal. Isobel overheard and blackmailed her stepdad into silencing her too.
Fisher goes to Prudence to gently ask if Edward could have been behind silencing the miners, when Dot screams, having found Birdie, dead in a puddle of milk, with eight toy cows. As Jack wonders how Quentin got in, Fisher realizes the shafts of the mine run under the house, and there must be a passage. They find it in the refrigeration room where the bodies are stored. But that's when Phryne sees Nicholas' shoes are soaked with blood. It's not Nicholas at all, but Quentin, his feet maimed to change identifying shoes. Nicholas, the real murderer, is alive.
In the cave, Fisher finds the gold seam, just as Jack is hit in the head by Nicholas. He admits this was all to get the gold and shoots at Fisher's feet to make her dance. But she attacks him just as Jack comes to in time to hold Nicholas at gunpoint. Nicholas confesses to murdering Laurie after the kid told him the men were alive. He set off the explosion, which killed the rest of the miners to hide his negligence. He never realized doing that would cause Edward to close the mine. This is his revenge a decade later.
With the roads melted, the family heads home to spend Christmas in July where they belong, at home. As Jane encourages each couple to kiss with a sprig of mistletoe, the episode ends on a happy carol. Merry Christmas, indeed.