This week's Grantchester begins with a family christening. It's bittersweet since last week ended with Will and his mother officially estranged. Once again, Vic at the boxing studio steps up with the advice. He suggests a date to the movies; he'll bring his wife Marie (Sandra Huggett), and Will will take Ellie. The boys from the studio come along as well, including Matthew and Lucas. The movie is interrupted, though, when the reel suddenly ends, and then by cigarette girl Betsey Granger (Zoë Tapper) finding the projectionist murdered in the booth.
Geordie: Hear that? The sound of splitting hairs.
Geordie and Larry had already stopped earlier to check out a reported a break-in and met the victim, Wyatt Rogers (Mark Umbers), a blacklisted American ex-pat. He showed them the projection room where someone had set up camp, but the vagrant had disappeared, so Geordie left them to run the next show. The usher, Edwin Hensler (Tom Morley), insists he saw the vagrant, an "Indian man" when he arrived for the evening show. He and Betsey admit neither worked there long or knew Wyatt well. But it's not a random killing. Among Wyatt's possessions, there's a Distinguished Service Medal for Aasim Hassan, who's rank, "Subedar," is from the Indian army.
When Will comes downstairs the next day, he learns the theater wasn't the only excitement. A brick flew through the Chapman's window during the night, after Jack spotted a young man hanging around the house. Jack won't report it, but Mrs. C's description makes Will realize it might be the same man seen at the movies. But by the time he gets to the station, Geordie's rounded up the suspect, Tariq Hassan (Hamza Jeetooa). He admits to living in the projection booth but swears he dropped his father medal when he heard someone coming and ran.
Will is puzzled why the kid traveled from Pakistan to Grantchester. There nothing connecting Tariq to Wyatt, though Wyatt's place is full of stuff leading to more questions, like a teddy bear on his bed, a typewriter beaten with a baseball bat, and expensive champaign bottles in the trash. He also seems to have had a lot of women over, with a pile of glamour photos, including Betsey's. She insists it was "early days" and kept it quiet because she feared upsetting Edwin. She also claims Wyatt promised to send her headshot to Los Angeles and fly her to America for a screen test.
Geordie brings Mrs. C in to identify the suspect, and Jack comes with her. She identifies Tariq as something wordless passes between him and Jack. They also confirm the window was broken at 8:15p, accidentally giving Tariq a rock solid alibi, since that's right when Wyatt was murdered. Jack then insists they won't press charges and goes so far to tell Geordie the kid needs medical attention, not a jail cell. With little to hold him, Geordie is letting the kid go when Mrs. C. shows back up and takes the kid out for lunch to find out what's going on.
Tariq tells her Jack was stationed in India as a civil servant during the Partition of India. He made his fortune driving families like Tariq's across the border who found themselves suddenly in Pakistan and vice versa. Tariq's father, Aasim, was waylaid, and Jack wouldn't wait. Tariq's mother paid him with her emerald engagement ring to bring Aasim. It never happened. Tariq is now trying to find any information on his father, and Jack ignored him. He apologizes for the window; he was just desperate. Mrs. C is horrified. She pulls off her wedding ring to give to him, saying she hopes it can make up for what Jack did. As she exits, Tariq realizes she's given him his mother's ring.
While at the theater, Will and Geordie catch Matthew stealing from the concessions till. Vic tries to claim it's his fault. The studio needs money; Matthew overheard him and Marie talking about it. Will is shocked, especially since Matthew tossed the cash, but Geordie arrests the kid anyway. Will puts money together and tries to pretend he found it in Will's locker hoping Geordie will let him go, but he's not buying it. Will goes to visit Matthew in his cell, trying to get out the truth but gets nowhere. In the end, Vic gets Matthew out, but whatever is going on is over yet.
While Geordie is off on the case, Cathy finds herself having to decide if she should let Diana take the kids for a day of hooky at the beach. She's wary until Diana reminds her of their adventures when Cathy was a kid. But they return home super late, and while Diana spins a yarn about what a good time they had, the kids' faces say otherwise. Esme pipes up Diana was behaving utterly embarrassingly in public, much to her grandmother's horror. Diana raises a hand to hit Esme, but Cathy stops it dead. Whatever Diana thinks she's doing, playing "Fun Grandmother," it just ended.
Meanwhile, Larry discovers Wyatt's sister lives just up the road. He's not American, just a kid from the children's welfare system who grew up to be a fabulist. But he's not a con man. Betsey says her headshot was free, and Wyatt let Edwin watch all the films. He even gave the kid a medal he found, thinking Edwin could pawn it. But even though the heirloom points to Edwin, the kid isn't the murdering type, falling to bits under questioning. He found Wyatt's body and put the medal back in his pocket. Geordie realizes it was Betsey who murdered Wyatt. When she realized his dreams of taking her to Hollywood were nonsense, she killed him for disappointing her, and now she's using his death to get locally famous.
As for Mrs. C, she's horrified to learn Jack got rich on blood money and he doesn't seem sorry. Worse, he gave her that ring as "an heirloom," a lie. At least Will appears to end on a happy note, with a second date with Ellie, while Geordie quietly takes Larry out for a pint in hopes of soothing a broken heart. But maybe Will's not ready for this to be more than handholding. Their makeout session sends him into a panic. He runs off, leaving Ellie behind, shocked and confused.