Leonard is gone from the Grantchester parish, but Will can't sit alone in his room and heads out for backgammon with Geordie at the pub. While there, they are accosted by Rachel Bromilow (Aimée Kelly) from the Student CND group, who is handing out fliers trying to end the proliferation of nuclear weapons. Will is naturally sympathetic, but Geordie insists that nukes ended WWII. But her protesting days are short-lived, as, on the way home, Geordie and Will spot her group painting a giant peace symbol on the dorms and tossing her off the roof.
Geordie: Someone should call the police!
The college porter, Bernard Allison (Alan Williams), is none too pleased with the mess. He says the kids were on the roof; he went up to give them what-for. The kids ran, Rachel screamed, and Allison looked over to her body at the bottom of the stairs. Her boyfriend, Rory St Clair (Neil Pendleton), says he ran the debate society and wasn't there, nor was their friend Josh (Akshay Khanna). Will heads off to find her roommate, Petra Neumann (Carlotta Banat). She was on the roof but ran in a different direction than Rachel and didn't see her fall. But everyone is hiding something because Rachel was stabbed through the heart before she fell and would have been dead by the impact anyway.
Rory and company turn their rage on Mr. Allison, who Rory says was a pervert who harassed Rachel. Allison claims he took her to the shed to calm down after she and Rory had a row, and he gave her "a cuddle" to make her feel better. Geordie finds the murder weapon in Allison's toolbox, a bloody screwdriver; he claims someone broke into his toolshed, stole the paint, and planted it. Geordie and Will believe the last bit — the handle's clean of prints, but the blood's been left on it.
Will agrees to pose as interested in the cause to see if he can get inside intel. He arrives to see Rory elected to head the group over Petra now Rachel is gone, forcing her to be secretary. Josh seems to be Rory's right-hand man at the meeting, but the two are almost immediately at loggerheads when it ends. Petra admits Josh was on the roof and has a talent for pushing people's buttons and he had a thing for Rachel. Geordie and Will pull him in, and Josh admits when Rachel called him up, he thought it was for sex. But he was the one who broke in and "borrowed" Mr. Allison's equipment to paint the peace symbol and claims he left to put it all back. Geordie realizes they saw him outside and can confirm his alibi.
The whole parish is grieving Leonard's exit. Even Dickens is sitting in Leonard's old room! (My heart!) But that room won't be empty long. The Bishop arrives on Sunday with Leonard's replacement, Henry Jones (Ahmed Elhaj), the first official Black actor to be a significant character on Grantchester. Will is furious he wasn't given a choice in the matter, which comes off far more racist than he means it to. Mrs. C is utterly crushed, realizing Leonard is never coming back, and she's not coming off too great either. That only worsens when she discovers he's a vegetarian; he insists on cooking for himself and wants to make his bed.
Rory, meanwhile, is now turning his attention to aiming the CND at the police in Rachel's name. Geordie has Miss Scott move them on, much to Will's distress, causing the two to have another row. As Will storms off, Geordie finds his old buddy, Johnny Richards, hanging out. The two have a drink as Geordie talks about the case, assuming he has a sympathetic ear who recognizes he's right and these anti-nuke people are evil. But Richards is far more of a nihilist, pointing out there's no right or wrong, just survival. Even so, Geordie invites Johnny around to dinner, where he charms the hell out of Cathy and Esme.
Will is down at the pub when Tamara comes in with her date, who is getting awful handsy. Not wanting to see his sister raped, Will break things up, claiming Tamara's his wife, to her utter delight. The two head off to hang out, drink, and steal a boat to have sex in. (What? It's not like they're related!) Will wakes the following day and regrets everything. As she leaves, she tells Will she's disappointed him, and a disappointed woman is a dangerous thing.
Will's not the only one regretting his life choices made while drunk. Geordie also clearly wishes he hadn't brought around Johnny and the questions around his wartime service. Thankfully, Will comes around to take his mind off it with the admission that he and Tamara may have done the deed (Will can't remember) and that Tamara may have solved the case. A disappointed woman is the most dangerous thing, and Petra has been deeply disappointed. First, losing out the leadership position at the CND to Rachel and then to Rory.
Petra had the murder weapon and knows who did it, but refuses to say who. However, her handwriting on the minutes for the debate club supplies Rory with an alibi. He was a disappointed person too, trying to apply to the Home Office for a job. Rachel got in, and he did not. Their fight over her staying home and being the good little wife rankled, and when she suggested perhaps he should stay home like a good little wife, he lost it and stabbed her.
But you know who else is disappointed? Henry Jones. Perhaps Will shall regret this all much more than he realizes.