This week's Grantchester begins with a young man running naked in the middle of the street. He passes Geordie and Will on their way to the pub for some backgammon, and Geordie nearly gives chase. But Will assumes it's "high spirits" among the college crowd. He's wrong. The kid is soon screaming about aliens as he passes by the wide-eyed old ladies, and by the next day, his body is found on the fens.
Geordie is livid, though more with himself that Will, but it doesn't stop him from taking it out on Ellie when she turns up, asking if this is the same kid seen running through town. But she's already tracked down the kid's identity: Chris Hartley. She gives the info up in exchange for dinner with Will.
Adler: What kind of neurosis leads a vicar to believe he's some kind of detective?
Chris was a student at Trinity Hall. His bedroom is a disaster, with books on the floor like "The Doors of Perception." His father, the widowed Mr. Hartley (Russell Wilcox), is stunned, describing his son as timid. At the Master's Lodge, Douglas Munroe (Andrew Bridgmont) and his wife, Dr. Judy Munroe (Stella Gonet), are horrified to learn of Chris' death. Dr. Munroe is a psychologist and Chris' mentor. She starts blaming herself, and Douglas attempts to shut down the questioning. But she insists; Chris was taking part in a trial run by Martin Adler, one of the college's "esteemed fellows."
Alder is studying experimental psychology. Judy sneers at the word "esteemed" and declares Adler's trials extremely dangerous. She says the experiments are using lysergic acid diethylamide, which the world will know in a few years as LSD. Will is perplexed that someone like Chris would consider the trial, considering how his notes worry about madness and death. Dr. Munroe says he asked for advice, and she told him not to. He did it because his girlfriend, Helen Fieldman (Sophie Walter), had signed up and pushed him into doing it with her.
When Geordie and Will head over to see Adler (Simon Kunz), he declares LSD to be a modern marvel, sneering Dr. Munroe is jealous because he got funded and she didn't. Adler even introduces one of his other subjects, Remi Adeyemi (Jyuddah Jaymes), who agrees LSD makes him one with the world, and the students only take tiny amounts. Geordie tracks down the lab making the LSD run by Professor Evans (Theo Fraser Steele). He insists it's tightly controlled. But his assistant, Peter Soosai (Akshay Sharan), under questioning, admits being part of the trial, and made an extra batch, enough for all four, stashed in Chris' room.
Remi confesses he was the one who tossed Chris' room looking for the vial when he heard about the running around naked. Helen is appalled when Geordie suggests Chris drank it, insisting Chris was frightened of taking LSD outside the lab setting, let alone by himself. Adler wants Will to understand what a wonder drug LSD is, and shows him a film of Chris' therapy, uncovering a memory of his dad killing his mother. Will, who sees abuse in every family, jumps to the conclusion that Chris' father poisoned his son with the LSD. But the elder Hartley disputes Chris' recollection. His wife was leaving him, and though they fought, that didn't happen. Chris' mother is alive and well, with three kids in Canada.
Defeated, Will pays up the dinner he owes Ellie and attempts to apologize for how last week ended. But when she tells him his behavior is humiliating to her, Will defensively points out she humiliated Chris by running his name in the paper. Ellie admits that maybe she shouldn't have, but the anonymous tip, "from a well-spoken lady," seemed harmless at the time.
Speaking of last week, Mrs. C is spending the evenings curled up in front of the TV in the vicarage, and snapping that nowhere in the bible does it ever say that God loves us and wants us to be happy. When Leonard gently asks why she's punishing herself, she breaks down, saying she married another crook with ill-gotten gains, just like her first husband. As Leonard tells Will what he's learned, mentioning he's thinking of contacting his own father, the vicar has a brainwave about punishing someone for "ill-gotten gains." Dr. Munroe was mad Adler got funding and wanted to humiliate and discredit his study, so she fed the LSD to Chris when he came to her for advice about doing it outside the lab. It's not murder, just tragedy.
With no arrests, Geordie comes home to discover Cathy dealing with an out-of-control Diana. She already stopped her mother from selling the family dishes and made calls looking for someone to take Diana in, but no luck. Instead, Diana invited the Lawsons, neighbors the Keatings haven't hung out with for decades, to dinner. Cathy attempts to let him Geordie escape to the pub, but he insists on staying and finds the Lawsons (and Esme) looking like they'd rather be anywhere else. At least he's there for the lighting of the Baked Alaska? (It goes just as badly as Iain's on Great British Bake Off, frankly.)
Diana's manic attack doesn't stop at dinner, and next thing you know, it's a sledgehammer to the walls. Geordie wrenches it away from Diana, telling her she needs help. Cathy, grabbing the opportunity, drags Diana upstairs. Realizing how ill his MIL is, Geordie asks Cathy why she never told him, only to have her write this off as "mum's ups and downs" before admitting the ups last for weeks....and the downs for months. She volunteers to quit Swinnertons, giving up her job to take back on four kids and now her mother as well. But Geordie refuses to let her throw away what she's built; what's wrong with Diana is too much. He calls the hospital and walks Diana to the orderlies in hopes she can get help.
Will is also looking for help. Between Geordie's irritation that Will sees family abuse around every corner and Dr. Munroe's suggestion that therapy and the church are not mutually exclusive, Will debates therapy. He runs to Vic for advice, but that's not helpful. Vic is well-meaning, but he doesn't believe in therapy. Instead, he tries to tell Will that marriages can be happy ones. But his story lands completely wrong, and Will takes this as proof he should ask Ellie to marry him. Blessedly, Ellie says absolutely not, telling Will to figure himself out. But that may be the end of that relationship.