Endeavour's Season 8 finale begins with the death at the end of the line for the 33 Bus to Chipping Compton, while newspaper articles from eight years previous splash headlines about a bloodbath at Tafferton Park. Morse is out, "sick" (read hungover AF), when Thursday and Strange turn up to see the body, Professor Patrick Stanton, stabbed to death with Xs carved into his eyes. Other than the bus driver, John Peckett (Ray Emmet Brown), and the creepy conductor Les Grant (Adam Ewan), the only passengers on the bus were Linda Travers (Anna Burnett), and an unidentified drunk who viewers already know will be Morse.
Thursday: "Bet he was good to his mother."
Frazil: "Kept her head in a hatbox."
Strange discovers the bus' clock-in box was disabled. Morse checks Stanton's office and finds a betting book, a grand prize winner flyer with stars over the winner's eyes like Staton's crosses, and a letter inviting him to Chipping Compton with "WYCH" and "WSW3MA?" on it. However, Thursday realizes Morse's regular bus is the 33, and his drunk ass was the last person to speak to Stanton. Frustrated and distraught over learning his son Sam's gone AWOL, Thursday gives Morse mandatory four weeks' leave to join a discreet Minnesota Model 12-step house.
Strange is riding the 33 Stanton took, but when Morse sees Linda Travers boarding at his usual stop, he impulsively hops on. It's the one after Strange's, the last one of the night, driven by Peckett and Grant. But the bus slides off the road as a snow storm rolls in, forcing Morse and the passengers to make for the nearest lodging — the now-closed Tafferton Park. While waiting for Strange, Thursday remembers what Stanton's eye crosses remind him of and calls Frazil to pick her brain over the Tafferton Park Massacre, where the now-dead Warren Loomis and Flavian Creech (Anthony Flanagan) killed multiple guests during a party. Strange returns; he saw Morse on the other bus, which is missing.
Trapped at Tafferton, Morse has accidentally stepped into Agatha Christie's Endeavour. The motley crew includes Travers, Peckett, and Grant; the latter suffered a concussion in the crash, looked after by passenger Dr. Gillian Nicholls (Jennifer Kirby). The other suspects include handyman Norbert Hobbs (Ben Bishop), solicitor Lawrence Yeager (Martin Hutson), former Exam board professor Percy Walsh (Matthew Marsh), pensioner Hilda Bruce-Potter (Marion Bailey), housewife Elsie Watson (Estella Daniels), and student Richard Blake (Adam Mirsky).
Travers says she was a Tafferton waitress and worked for Churchyard, the victim who had his eyes Xed out. She leads Morse to the generator and then looks for supplies. When Morse returns, Peckett and Hobbs have gone to find food and alcohol, respectively, and the latter's disappeared. Walsh takes an interest in Morse, especially after he openly goes through Hobbs' coat. (There's another betting book and letter that match Stanton's.) He insists on helping look for Hobbs. But Travers finds him, eyes Xed out, dead in the ballroom still set from the party all those years ago.
With another death, Morse outs himself as a detective. Hobbs is wearing cufflinks that match Stantons; Churchyard's office also has a set and a telegram with the "WSW3MA?" code. Morse also notes a ballroom table has the name cards Yeager, Walsh, Stanton, and Hobbs, giving viewers the next victim to expect when Yaeger wanders off. However, Peckett is unimpressed, calling Morse a useless drunk, and gets Walsh and Nicholls to check on Grant with him. Left with a too-chill Bruce Potter and a sleeping Travers, Watson runs for it just before Nicholls, Peckett, and Walsh return to report Grant is gone; Peckett gives Blake a drink to steady him.
Thursday heads home to find Win grieving her missing son as Joan tries to keep the peace. Frazil shows up with the Tafferton files. Her research suggests Creech did the killing for Loomis, who committed suicide rather than stand trial. A visit to Creech confirms this; Loomis was "numbers not wet work," Creech says Churchyard had the kid put away to cover they stole millions from him. He also nods to a picture on the wall of Loomis with his sister and a guard, noting Loomis' grandmother also visited. The sister looks like a young Linda, and the guard like Peckett.
Yeager turns up dead in Churchyard's office, so Morse sneaks Nicholls out to go for help and then returns to find Walsh trying to break into the safe. Walsh tries to play dumb but confronted with WYCH standing for Walsh-Yeager-Churchyard-Hobbs and WSW3MA as the "Witch" phrase "When Shall We Three Meet Again," he breaks. Loomis was an autistic numbers prodigy student of Stanton's. The group was part of a betting pool where Loomis could pick out the winning patterns. They used him to win the grand prize of £125k, and then Walsh had him committed to keeping from anyone finding out. The winnings are in Churchyard's safe; their cufflinks hold the code only when all five are together. That was to happen the night of the party; instead, Creech brought Loomis there to enact revenge.
Watson reaches the road just as Thursday arrives. Grant's on the bus, where Nicholls is trying to save him. Morse and Walsh leave the office to confront the killers finishing Creech's work, the only three left: grandmother Bruce-Potter, sister Travers, and sympathetic guard Peckett. Thursday rolls up just before the three kill Walsh and then turn on Morse to protect him. Everyone is arrested, including Walsh; Morse promises he'll be put away for the longest.
In the light of the morning, everyone who died deserved it. However, Morse, having been forced to stay sober all night, admits he has a drinking problem. One assumes he'll return in Season 9 a bit soberer and perhaps finally on the path that will make him the Morse we met back in 1987.