Will was in a lousy headspace at the end of last week's Grantchester. The finale opens with him in a worse one, drunk and surly, having crashed his mother's engagement party to St. John.
Only Clement (Jonny Amies), his teenage step-brother-to-be, gets a kind word. Amelia, horrified, doesn't throw him out, but she does distressedly snatch back the champagne he's downing. Will wakes the next morning with a hangover from hell, hoping Leonard will take over services. But Leonard's father, Russell (Sean Gilder), is on his way, making this the last thing he needs.
Grace: Happy birthday! How old are you? Actually...I'm not sure I want to know.
It's been up to Geordie to keep an eye on Matthew, who's now homeless. He's also back to solving crimes with Larry, the latest being a woman, stabbed to death, and then laid out, crucifix-style, on Jesus Green. According to the marriage certificate she carries, her name is Nora Martin. The religious aspects are a perfect reason to round up Will, who goes with him to see Nora's husband Frank (Adam Astill) and son Robin (Harry Gilby). Frank tells them Nora's been missing for five years, believed dead. Looking her over, Will realizes Nora was a nun, and the engraving on her Bride of Christ "wedding ring" is the St. Beatrice motto.
They're let in by Sister Michael (Andrea Valls), who takes them to the head of the order, Sister Grace (Tracy Ann Oberman). Grace shows little emotion over "Sister Elenora," though Sister Michael admits Nora seemed "out of sorts" before disappearing after Friday morning prayers. But then Sister Francis (Adeyinka Akinrinade) initiates the call to prayer, and Sister Grace shuts them out. Back at the station, Larry's pulled the missing person cases from five years ago. It's got Nora's file, but Will seizes on the next one, Lavinia Jones, unmistakably Sister Michael. Mr. and Mrs. Jones (Mark Gillis and Sarah Thom) knew their daughter ran off, and agree she was a religious sort. Photos of her confirmation include Sister Grace.
The Jones' confirm Grace was ex-communicated from their church. Will goes to see Sister Michael, who freezes at being called Lavinia and shocked her parents are still looking for her. Francis runs up to pull her away, insisting, "Sister Grace loves us." Convinced they're are brainwashed and kidnapped, Geordie stages a raid, reuniting the girls with their families. It also turns up Hattie, Nora's five-year-old daughter, who Frank didn't know existed. But Grace insists her convent was a loving family, and any "evil" Will sees is projecting his own issues.
When Lavinia arrives at the station, beaten, and begging not to go back to her parents, Will realizes Grace was telling the truth; these were abused women. They head to the Martins for a welfare check, and Will realizes the rug's been changed. Frank sees them roll it back to reveal a bloodstain underneath, and confesses to murdering Nora. But Frank's description of the murder is wrong. Will realizes Grace said Nora was going to the school to register Hattie, which Robin attends. Nora saw him and couldn't help herself, reaching out to explain where she'd been. He killed her in a rage that she'd left him, acting out the same attitudes Frank only expressed as emotional abuse.
Having gotten it wrong (again), Will apologizes to Grace. She tells him he reminds her of the abused women who turn up at her door; someone frightened of himself. Will admits he's terrified of not being perfect, and if he falls, he'll never stop. She promises when she fell, that didn't happen, and the more he resists his feelings, the more unhappy he'll be. Then she beds him. Geordie's going to be so tickled to hear Will got laid and on his birthday of all days. Ellie might not, but she does agree to try their relationship again.
But the real heart of this episode is Leonard's story. For all Daniel's pep talks, Leonard reverts to nervous stuttering at Russell's arrival. Jack talks up Leonard, but Mrs. C thoughtlessly mentions Leonard's lack of a wife, giving Russell an opening to sneer how disappointing Leonard is as a son. He takes off for the pub rather than see Leonard's sermon. Afterward, Leonard finds him drunk, talking about his old pub friend Gideon and refusing to discuss Leonard's late mother. Russell winds up trashed, crying he was a disappointment to his wife, and going on about Gideon. Leonard is distressed, shutting the conversation down. But Daniel is more clear-eyed and puts the pieces together.
Leonard is stunned at the idea his father is closeted. But when he confronts Russell, he realizes Daniel is right. This man who emotionally abused him was secretly gay, cheating on his mother. Leonard becomes enraged, calling Russell disgusting. Daniel is shocked by Leonard's response, saying maybe the apple hasn't falled that far from the tree. Leonard attempts to make amends as Russell leaves. But Russell's response is to escalate, threatening to tell the church Leonard is gay. At that, Leonard finds his righteous anger, throwing Russell out. He's happy for the first time, and his father will never know that feeling.
As Leonard makes peace with his choices, Geordie goes home and asks Cathy if she'd like to see her mother. He harangued Will to see Matthew; maybe they should visit Diana. Meanwhile, Will goes to see his mother too. Amelia is glad to see him sober, telling him not to go back to he was before he found the church. She wants him to be happy. He says he only wants the same for her, and he's afraid she'll be trapped with St. John the way she was with his father. But Amelia's not trapped; this one has money. And Will suddenly realizes this could help him too.
The Gurney-Clifford cash is to bankroll reopening the gym. But Will doesn't plan to do it alone. He rounds up Matthew, offering it as his place to build to help save boys like himself. It's the best birthday present he could give to anyone, especially himself, better than the Marlon Brando photo Leonard gives him or the Keating clan's motorbiking gloves. But there's one more gift. Daniel walks in. Leonard tries to act like he shouldn't be here, but Mrs. C steps up and says *she* invited Daniel, as a friend of Will's, he's part of the family, as everyone smiles, knowing this isn't about Will at all. Perhaps this season, Leonard got the best present of all.