The final episode of Magpie Murders is packed with revelations from the Alan Conway whodunit and the story of what happened to the author himself. In a change of pace, instead of the story starting in the real world, either present-day or a flashback, this week's finale begins by picking up where viewers last left off in "reading" Conway's final novel, which itself is a flashback scene to seven years previous, just as the flashback scenes that opened Episodes 3 and 5 were.
The Blakiston boys burst into the lodge house screaming to their mother that they can't find their dog; Mary suggests they try Dingle Dell. But when they arrive, Brent stands by suspiciously as the brothers come upon the poor pup dead in the woods. Meanwhile, flashing forward to "the present day," Pünd and Fraser visit Matthew Blakiston, whose family was profoundly affected by the actions of Sir Magnus Pye. The elder Blakiston provides Pünd with a few essential facts, including the ill-fated puppy, Bella, was a birthday present from Matthew to Sam and was presumably poisoned by Brent.
As for the drowning of his dearest son, Matthew's retelling of events includes Robert rushing into the water to get to his little brother as Brent was carrying him from the lake. In the car, Pünd declares Matthew has given him the last pieces of the puzzle. Once they meet with DI Chubb, they can return to London.
Susan concedes defeat; the chapter containing the resolution of Pünd’s case for the incomplete Magpie Murders manuscript is nowhere to be found. Meanwhile, Charles has moved to recruit a new CEO and hired author Sophie Hannah, who has written four new Poirot mysteries since 2014, to finish Conway’s book. Hannah agrees the title should be The Magpie Murders, not just Magpie Murders, as Alan insisted, prompting Susan to look closely at all the Atticus Pünd novel titles; lo and behold, Alan’s penchant for wordplay rears its pretentious head. The first letter of each title spells out “AN ANAGRAM,” with Magpie Murders as the final M.
Andreas gets a tech-savvy colleague to help him trace the email with the incriminating photo. It turns out it originated from Clover Books. Meanwhile, Alice mentions to Susan she ran into former co-worker Jemima (Kate Gilmore); Susan gets her number and calls. Jemima reiterated she copied the manuscript and checked the page count. However, she says the manuscript came on Wednesday morning. Charles said he got the manuscript from Conway Thursday; she was laid off Friday. Susan returns to the office and finds the missing chapter of Magpie Murders. It’s been in Charles’ desk the whole time; he enters, realizing the jig is up.
Charles read the manuscript before Thursday’s disaster dinner with Conway; the author couldn’t hide his glee at killing Pünd. Despite a handwritten apology from Alan, Charles drove to Suffolk to make his client see sense. When Charles arrived, Andreas was pulling up at Abbey Grange, so the publisher pulled his car out of sight, leaving tracks at the side of the house. Clover took the photo of the two men on the tower and sent it to Susan, thinking she would stop investigating if Andreas was involved.
Charles was facing ruin just because Conway resented writing insipid whodunits. Once Alan discovered he was dying, he decided he could finally bring his legacy crashing down. The anagram Conway's book titles referred to was Atticus Pünd’s name: "A Stupid C*nt." It would have ruined the reputation of the series and the CityWorld Media deal. After trying to reason with Alan, Charles pushed the arrogant writer off the tower. Susan also surmises that the clear-out of Alan’s office was necessary because Charles had seen something in the manuscript that he could use, Pünd’s suicide note at the end of the book.
The handwritten draft of that note in the final chapter combined with Alan’s apology letter would suggest to the police that Alan’s death was self-inflicted. The reused, typed envelope containing a handwritten letter was the giveaway that Susan couldn’t ignore. Charles asks if Susan is going to turn him in; she suggests he do it himself. He asks for the evening to spend with his family before surrendering, and then clobbers Susan on the back of the head with a clunky publishing award, sets the office on fire then leaves the building.
Susan comes to and tries to salvage the manuscript from the flames, but the smoke overwhelms her. Pünd is seen walking through the inferno, but actually, it’s Andreas, pulling Susan from the fire just in time. At the hospital, Katie breaks the news Charles was arrested and their dad has passed away. Susan apologizes to Andreas for thinking he could have killed Alan and says she wants to go to Crete. However, her final caller is Pünd, who compliments Susan on her excellent detective work and invites her to come with him to find out the solution of Magpie Murders.
Joy Sanderling, her fiancé Robert, James Fraser, and DI Chubb have gathered at the Queen’s Arms as Mr. Pünd arrives. He begins with Mary Blakiston’s death, an accidental tumble down the stairs, brought on by her haste to answer a phone call from Lady Frances Pye. As for the murder of Sir Magnus, Atticus says they must go back to the death of Robert’s brother, Sam. Robert didn’t just harbor your garden-variety jealousies but a disturbed mind. He poisoned the dog and drowned his brother in the lake. Robert went splashing into the water to meet Brent to cover up the fact that he was already wet.
Mary Blakiston witnessed Robert’s heinous act from the lodge house and afterward kept extremely close tabs on her surviving child. Her refusal to give Robert and Joy her blessing to marry was more likely to keep her son from spreading his violent instability to future generations rather than a racist rant. Mary wrote a letter recounting Robert’s murderous behavior, which was sealed and kept in Sir Magnus’s safe, only to be opened in the event of her death. At her funeral, Robert discovered the existence of her letter, so he tried to steal the damaging evidence while Sir Magnus was on holiday.
He covered up his failed break-in by staging the burglary, then returned to meet with Sir Magnus, asserting that his mother got her facts wrong. When Lord Pye refused to be swayed, Robert beheaded him with a sword and went back to the study for his mother’s letter, burning it in the fireplace. The envelope holding the letter remained on the desk, and Pünd could match it to the handwriting from Mary’s diary. Robert is taken into custody by Detective Chubb as a devastated Joy looks on.
Susan and Atticus stroll across Dingle Dell as she praises the detective’s prowess; she’s sorry they won’t meet again. Pünd says they have separate paths to follow, but it’s been a pleasure. As Susan wanders off, Atticus tips his hat to the mischief of magpies perched nearby. A bittersweet ending to one usual friendship between an editor who loved the certainty of mysteries and the beloved character who mentored her.