Grantchester opens its new season with what seems like paradise, Will Davenport, the young squire of his family, in tie and tails, wining and dining the fresh-faced young ladies from Newnham at Linley College's Spring Ball. But outside, DI Geordie Keating is in the car with DC Larry Peters (Bradley Hall), waiting for the drinking and the fistfights to begin. They're not the only ones with their eyes open to the reality of how these things go. Ellie Harding (Lauren Carse), an enterprising reporter, has commandeered the closest phone booth with the gossip.
Ellie: Oh, you're that Will? Should change your name to Won't.
But the gossip is minor compared to the body that washes up after the party. It's a Newnham student, Jessica. The coroner says she was dead before she hit the river. Her fellow students beat a hasty retreat when they see the cops. Will notes, since Linley is a boy's college, they can't get caught. But Veronica (Sorcha Groundsell), Jessica's BFF, does step forward, saying her pal disappeared and wasn't back at the dorm when she signed in at 11 p.m. Geordie pushes to know who they were dancing with and how much they drank. However, Veronica is more obsessed with the other girls.
Andrea Shaw (Siobhan Redmond), the head of the college, insists girl-on-girl bullying is a tired old trope and not tolerated. But Shaw is turning a blind eye, ignoring that queen bee Heather (Hannah van der Westhuysen), and her friends all signed with the same handwriting. Looking to find the truth, Will has the idea to check the Linley photographer's negatives for the final party photo, taken the next morning to prove who made it. They pop round to Daniel's, who processes it for them. Not only is Jessica in it, but so is Veronica, despite her 11 p.m. sign-in.
On their return, Matthew Butler (Jim Caesar), one of the waiters, seems too interested in Will and Geordie's presence, and it's soon clear why. He was crushing on Jessica and saw her and Veronica go upstairs with two Linley students, Alastair (Abraham Lewis) and Ollie (Barnaby Taylor), and believes they murdered her. Will notes they had motive since they were expecting sex, and the girls refused. But Alastair reveals the girls were part of a drinking club: The Valkyries. Shaw knew and tacitly approved. But if she thinks these girls aren't bullies, she's fooling herself. Heather is all too eager to sneer Jessica "wasn't as discerning as the rest of us."
Considering how they parted at the end of Season 4, it will surprise viewers that Leo and Daniel arrive back in Grantchester together, having just gone on vacation to Morocco. But though it was paradise, it's also a secret. No one knows they went, and all Leo is willing to do now they're home is a stiff handshake. Also, Leo lied to Mrs. C, telling her he was going to the English Rivieria in Bognor, but foolishly brought back presents obviously from the southern coast. Will tries to ask him about it over lunch, but with Mrs. C there, Leo sadly can't tell the truth.
Will asks them to distract from his mother. Amelia had taken to coming round, bringing endless complaints about her far less posh life, as Will pushes her to make a fresh start. Mrs. C tries to tell Will that's not how it works, he can't expect his mother to live as he assumes she should. Meanwhile, Ellie comes to hang with Will and Geordie to get information, and, much to Will's horror, tricks him into confirming details. He calls her a gossip monger, but she gives it right back, telling him at least she doesn't spread lies from some old book.
But Ellie does have her uses, digging into Shaw's history. She also suggests they go to the Newnham Spring Ball. Even Will admits it's a pretty good idea. With his posh connections, he gets himself and Geordie on the list. It's an excuse to get dressed to the nine, though Will's quiet putting together is far sexier than Geordie, who's house is a disaster with both him and Cathy now having respective full-time positions. We also get to see Will dance with Veronica and Geordie turn down a dance with Heather, before stopping Shaw when he sees her discreetly passing things out. But they're not the pep pills. She's passing out condoms, bless her.
Veronica spots someone in Jessica's room, and when Will gives chase, it turns out to be Matthew, retrieving his dad's cufflinks. He admits he gave them to Jessica to use as a trophy: To join The Valkyries required sex and items to prove it. Why him? Heather noted his crush and thought it funny to haze the wannabe by forcing her to sleep with someone lower class and of color. Will notes it's an American cufflink — Matthew admits his late father was an American GI — and realizes Matthew's the pep pills supplier, getting them from the nearby base.
Shaw is horrified at the hazing, but Heather isn't having it. Shaw can preach careers all she wants, but soon enough, they'll be married with kids, organizing charity lunches. This is their Eden, and the only fun they'll have. But it's not fun for those trying to join the gang, it's a desperate game, driving girls to make poor choices, like going up to Alastair and Ollie's. Jessica left, but Veronica, defenseless, wound up raped. She lashed out when she found Jessica, hitting her with a bottle in a speed-fueled drunken rage. Shaw tries to stop Geordie from arresting Veronica, insisting Ollie and Alastair's actions drove her mad. But Geordie knows no court will convict them.
The rich boys walk free, but Matthew will be arrested too for drug dealing and withholding evidence. Desperate to make something right, Will convinces Geordie to let the kid join Vic's boxing halfway house for kids. And while Geordie's at it he'll fix another problem, calling Cathy's mum, Diana, to come help with the kids. Cathy's face, when Diana walks in, suggests Geordie may have just added new problems instead. As for Will, his mother is coming round, though not for the reasons he'd hoped. She's met a new man who "doesn't like to use his title." She'd love Will to meet him, maybe in his estate out in Norfolk? A return to her paradise is a return to the life she lost. Let's hope this one isn't as abusive as her last husband.