The mystery plot is particularly good in this episode. And it's all about the vicarage. Maybe the location was busy for the first two episodes of this season; they're very much back to services, with Tom Brittney giving a couple sermons. He's also potentially hiring a new curate (Tom Glenister, briefly returning from last episode). Brittney takes Glenister to visit Al Weaver, which is just set up for Tessa Peake-Jones's subplot. She still hasn't told anyone but Weaver about her cancer diagnosis.
"Grantchester"'s got alternating C plots this season: there's Peake-Jones and cancer, then Robson Green and Kacey Ainsworth's marital woes. This episode starts with Peake-Jones as the main subplot, switches over to Green and Ainsworth, then is back to Peake-Jones for the finish.
It balances out nicely—with some great acting from Peake-Jones and Nick Brimble (as her husband)—because the Green and Ainsworth subplot echoes in the main mystery plot.
The victim this episode is the husband of one of the church ladies, who Brittney thought he knew well, only to discover they've all got secrets from him. Except for Oliver Dimsdale (Weaver's beau), the lone dude in the group. Louiza Patikas plays the controlling leader of the group, with Lauren Douglin and Sophie Fletcher as her chief sidekicks; Alaïs Lawson is the youngest, a French girl who Patikas micromanages the most.
Brittney thinks he knows all of them, only for the murder to reveal he's completely unaware of what's going on with his parish. Green's even surprised at how out of touch Brittney is with the goings-on amongst his most needful "customers." Though at this point, there's no longer mention of Brittney not hanging around the station to help Green with investigations; Green's new boss doesn't put in an appearance this episode (or last), and junior officer Bradley Hall is back to hanging on Green's every potential compliment.
The mystery reveals things about the victim, the victim's wife, and the three other women in the church group; lots to unravel, lots of well-acted drama. Patikas proves a deeper character than implied initially, and she's terrific. The murder arc ends up far more haunting than it originally seems like it'll turn out.
It also ties into the Green and Ainsworth subplot. Brittney and Charlotte Ritchie are convinced all Green and Ainsworth need is a nice night out and set them up, unknowingly, on a double date. Ritchie realizes Brittney sometimes defaults to vicar behavior when things go wrong and right for certain attendees, making him less than sincere when he needs to be. Unfortunately, the episode leaves that development hanging for next time, instead toggling back over to Peake-Jones's. It's an okay move (the Peake-Jones subplot does take precedent), but they seem to be dragging out Brittney and Ritchie's flirty friendship.
Even with the frayed subplots, it's another good episode. Though it's only a six-episode season this time, so they're nearing the finish, and almost everything outstanding is very outstanding.