A brief montage of scenes opens Season 9, the final season of Endeavour. Musicians and audience members gather for a concert by the Oxford Concert Orchestra, back in town after an extensive tour abroad. Sir Alexander Lemertov (Nicholas Farrell) conducts Mendelssohn with violin soloist Christina Poole (Kirstin Louie). A man is pursued down an alley, stops briefly in a phone box, and then is captured and tortured.
At the Thursdays’ home, Win polishes the frame of a photograph of their missing son Sam (Jack Bannon). And Endeavour Morse (Sean Evans) is back in Oxford, sitting peacefully by the river doing a crossword. Some things don’t change in the city of the dreaming spires, or do they?
Morse is back in Oxford following his alcoholism treatment, which DCI Fred Thursday (Roger Allam) and CS Reginald Bright (Anton Lesser) discuss in hushed tones. Bright, planning to retire, offers Thursday a promotion but at a different station. Word in the department is Morse had an extended holiday in the west of England, and he tells DS Jim Strange (Sean Rigby) he’s been following Hardy’s footsteps. Strange responds with Laurel & Hardy, “There’s another fine mess.” (Maybe he knows Morse is referring to Thomas Hardy and is winding him up.) In any case, the only notable difference is Morse no longer goes to the pub.
Morse joins Thursday and Dr. Max DeBryn (James Bradshaw) in an Oxford college garden where a body has been found. He’s wearing only one shoe, an expensive brand from Burridge’s Department Store; the time of death suggests he was killed after the college's reception following the Oxford Concert Orchestra’s concert and dumped. Morse finds an ID bracelet with the initials AL as he leaves. (Why didn’t the tech team find it? Were they too busy looking for the second shoe?)
Morse interviews the orchestra members, who haven't a hint of harmony. They even bicker over the biscuits served by tea lady Pat Treadle (Jenny Galloway). It’s a hotbed of sexual intrigue and jealousy from Lemertov down; the musicians were not invited to the reception (they went to the pub). It was mostly Oxford dons, in honor of the prestigious Belasco Academy and its founder Madame Belasco (Jane Lapotaire).
Lemertov, angry that a rehearsal has been interrupted, recognizes Morse as a singer in a choral performance he conducted, making a derogatory comment about amateurs. As Morse talks to orchestra manager Donald Fischer (Rob Ostlere), soloist Christina Poole (Kirstin Louie) engineers tripping on the staircase. Morse, meeting a pretty woman who is a musician, responds like a cat to catmint; Donald also clearly has a crush on her.
Christina wants Morse’s advice (someone’s been writing derogatory terms on her dressing room mirror) and invites him to the next day’s concert and awards ceremony at the Belasco Academy, a prestigious boarding school where Christina and others trained, including first violinist Margeaux Quincannon (Tamsin Newlands) and her desk mate Mabs Portman (Imogen Daines).
Lemertov watches a video of himself pontificating about the mystique of the Ninth Symphony. Sadly no, it’s Lemertov’s Ninth, an unpleasantly atonal work. Margeaux collapses on stage during the opening bars. DeBryn, in the audience, responds, but Margeaux is dead. The word whore is in red lipstick on Christina’s dressing room mirror, matching Margaux’s. Christina receives a second nasty shock of Lemertov waiting in her hotel room, jealous of Donald and claiming he made her the musician she is. She asks him to leave.
DeBryn says she died of anaphylaxis due to a severe nut allergy, but Morse knows she wasn’t carrying adrenaline, as a person with severe allergies would. The interviews reveal that Margaux jealously protected her position as First Violinist (the orchestra's leader and highest-paid musician). Violinist Lindsay Trench (Leaphia Darko) says Margaux and second violinist Mabs are “two peas in a pod,” and violist John Graham-Scott (Nicholas Boulton) says Margaux used her sexuality. Both claim she had “a thing” with Lemertov.
Mabs, who has been close to Margaux since attending Belasco, denies it but claims Lemertov took a particular interest in Margaux’s career. She also says Margaux had no other close relationships, despite the draft of a threatening letter found in her room that reads like a letter to a former lover. Lemertov denies anything improper, claiming his relationship with Christina is on a far higher plane –– she is his instrument (ugh). Even kindly tea lady Mrs. Treadle admits that Margaux was difficult and fussy about her biscuits.
Morse, Thursday, and Strange are called out to another murder; a man found mutilated and tortured to death in a warehouse. It’s Mickey Flood, Thursday’s former London informant, who had come to Oxford specifically to see him and had recorded his phone number on a matchbook, probably taken from the directory in the booth outside the warehouse. The directory is still open on the page where Thursday’s information appears. Thursday is concerned that London gang warfare may spread to Oxford, and after 25 years of silence from Mickey, it can only mean something dire is afoot.
Meanwhile, Morse is inquiring about the shoe at Burridges, an old-fashioned family-owned store with more protocols than Oxford. Granted permission to view the Big Book, he finds the identity and address of the victim, Andrew Thompson. Morse discovers on the same page Joan Thursday (Sara Vickers), with whom he’s still in love, returned a pair of gloves to the wedding department. Back at the morgue, DeBryn discovers Andrew was murdered, heroin and barbiturates injected into his foot. When Morse and Thursday visit Thompson’s apartment, they find signs of a struggle and the other shoe, discarded post-injection. Morse also finds a business card for Ace Private Enquiries.
Ace Private Enquiries is run by former cop Ronnie Box (Simon Harrison). Ronnie confirms Andrew was a client, but his last name was Lewis. He was trying to discover what happened to his mother in 1966 (Season 2) when the Blenheim Vale child abuse scandal broke. Andrew’s mother worked for Landesman Construction Company, owned by Josiah Landesman, who seemed to be at the heart of a web of child abuse centered on the Blenheim Vale children’s home. High-ranking police officers and prominent community members were involved, but the case was never fully solved since Landesman disappeared, probably having fled abroad.
Thursday decides to pursue that case while Morse continues solving Margaux’s murder, as he and Bright fear for their station’s legacy and the city's safety if the London mob is involved and if there are links with the Blenheim Vale case. Morse visits Madame Belasco, who says Mabs and Margaux were a year apart and not close until Mabs changed her instrument from viola to violin. Margaux’s best friend was Rose Garland, an enormously gifted musician whose death in a drowning accident devastated Margaux.
Morse finds a hotel bill that shows Christine had an adjoining room on tour to that of fellow-musician Fergus Brathwaite (James Anderson). Under questioning, Fergus says Mabs was behind all the backstage nastiness and admits to dumping Margaux for Christine. (Lermontov would not have approved.) Margaux threatened to leave the orchestra and expose Donald Fischer for embezzling. Asked about Fergus’ claims, Mabs says Margaux developed obsessions, and Donald was innocent. Donald makes vague claims of creativity in his record-keeping, which doesn’t go well with Thursday and Morse. It’s the foreign tours, he explains, that keep the orchestra in funds for performances in Britain. He also claims Margaux was behind the lipstick messages. He was going to confront her, but she died before he could.
Morse looks up Rose’s drowning and discovers Belasco Academy was a toxic environment; Rose was a scholarship pupil, bullied by the others. Rose lived with the Treadle family after her family died during World War II, and she came to the area as an evacuee, though she was never formally adopted. Madame Belasco denied bullying occurred, but Mrs. Treadle still has the anonymous note she found in Rose’s pocket, telling her to kill herself. The coroner couldn’t rule out suicide and gave an open verdict. Margaux, Mrs. Treadle says, was a lovely girl and devoted to Rose.
Thursday is trying to persuade Win that a move and promotion will be in their best interests now Sam is home, released from military prison, and full of resentment and anger. It’s not a happy homecoming. He finds it impossible to make his father understand what serving in Northern Ireland was like, and Thursday, like most of his generation, can’t talk about feelings. Looking for Joan, Sam finds Morse, who offers him a ride to her place. Morse and Joan have a stilted conversation, and he mentions the bridal gloves, thinking she’s a bridesmaid, only to learn she’s engaged to Strange. At the office, Morse congratulates Strange, who asks him to be his best man.
Morse reacts as he always did –– ordering a pint a the pub. Watching a customer eat peanuts, he comes up with the idea of ground nuts being added to Margaux’s rosin by someone borrowing it. The transfer of the rosin from the bow to the strings proved lethal. He takes the violin to DeBryn for tests, and dust on the violin tests positive for nuts, but her rosin is clean. It seems obvious someone jealous and wanted revenge is to blame; someone who is second best, second fiddle: Mabs. Mabs is arrested. She tells Thursday and Morse she was devoted to Margaux and destroyed Rose’s suicide note. Margaux couldn’t forgive her but couldn’t let her go; this terrible secret locked them together.
Post arrest, Thursday finds Morse dozing in the pub, drunk, claiming “everything in moderation” but as sharp as ever. He’s figured out Thursday offered to clear Mickey of racketeering charges in exchange for information. But why did Mickey want to deal with Thursday, not an officer in London? There’s a lot unspoken as Thursday leaves to return to his family, leaving Morse alone, trapped in his own thoughts.