The magic of Grantchester, and any crime drama on Masterpiece of its ilk, is that it knows its limits in terms of knowing when to focus on the crime and when to focus on the characters. The show does not normally veer off into the wonderful world of horror, but in episode 4 of season 10, Grantchester almost did just that.
Caution: This post contains SPOILERS for Grantchester season 10, episode 4.
Part of the joy of Grantchester is no crimes bleed into the next episode. Almost always detective Geordie Keating and his partner-in-crime-solving, the good vicar Alphy Kottaram, resolve whatever the mess happened to be and move on to the next violent crime.
This makes every episode sort of a standalone, but the string from episode to episode is the characters and the relationships they have. They might have seen some bad stuff together, but they normally only discuss interpersonal things.

In Grantchester season 10, episode 4, we learn a lot about Alphy (and so does Geordie)
This is important to note in episode 4 because we learn something about Alphy we didn't know before. Neither did Geordie and that all ties into where the crime occurred.
At the beginning of the episode, Alphy plans on having a relaxing day (like that ever truly happens in the village of Grantchester) until he gets a mysterious call from someone he clearly knows and needs help. He leaves the rectory without saying where he is going.
He arrives at an orphanage where he is greeted by Stuart Potts, who has obviously known Alphy well in the past. Stuart explains the orphanage is closing and the children who live there will need to go to foster homes. Stuart has called Alphy there to talk with the kids and reassure them life is going to be fine.
While talking to the children, Alphy is standing in front of a chalkboard. He turns it over to present some more information only to find a doll stuck to it with its forehead painted red as if it was bleeding.
Things get really creepy
While that is surprising enough, we soon see Alphy in his room later that night, only to be awakened by the sound of children chanting outside their rooms near a stairwell. If you had never watched Grantchester before and turned the show on at this exact time, you might have turned it immediately off because it was an exceedingly creepy scene.
Nurse Butcher (which is a fantastic name), who helps Stuart run the orphanage, tries to get the children to stop, but they suddenly stop on their own and return to their rooms. To make matters worse, later Alphy hears a thud on the stairwell, only to leave his room and find a boy named Joshua has either fallen down the steps or been pushed. Either way, Joshua is clearly dead and lying in a pool of blood.
The local detective is called but is useless and refuses to investigate the crime. You can likely guess that the next thing Alphy does is call his good friend Geordie. Stuart is of no help, either, though as he doesn't think any of the children would want to hurt any of the others and believes Joshua must have tripped.
Alphy later finds a Ouija board with loads of candles around it at the bottom of the steps where Joshua was found, and Alphy and Geordie question a girl named Joyce, who earlier in the episode had given Alphy a candle. Joyce says not to look at her as she didn't do anything but go ask another girl named Rita about it.
That is when Rita spills the proverbial beans that she advised Joyce to use the Ouija board to try to determine the fates of the children since the orphanage is set to close. Rita also suggests that Georgie and Alphy go look at the notes that Stuart has made about the children because maybe Stuart and Nurse Butcher want the children to disappear.
Alphy refuses to believe this and explains to Geordie that he has faith in Stuart because Alphy grew up in the orphanage himself after being left at the doorstep. Geordie is as shocked as viewers are by this news, but the detective does his due diligence of searching Stuart's notes. While they are speaking, though, a boy named Oz is heard muttering on the stairwell but as he is falling Geordie catches him.
At this point, Alphy and Geordie have to be asking, "What the heck?" as something is clearly amiss.

Alphy and Geordie learn the truth
Oz, who had passed out, wakes up and explains he was given some kind of potion that was supposed to induce hallucinations, including one where he would meet his soccer heroes. Geordie had previously seen Oz kicking a soccer ball around outside.
The detective and vicar go searching for what might have been in the potion only to find some flowers in Nurse Butcher's room. The flowers are wolfbane and deadly, but Nurse Butcher says she does not know how they got there. She does suggest that Rita might have an idea because Rita believes Butcher and Stuart hate the children and Rita would do anything to keep the orphanage open.
As Geordie and Alphy go to question Rita, she tries to run away but they stop her. She then explains she was hoping to give wolfbane to everyone so they would die and then she would take it and meet everyone in heaven.
Again, Grantchester went nearly full horror in this riveting episode and they did a great job of it. Rita is later arrested and Geordie explains she will be going to jail.
Oz was about to be adopted before the death of Joshua but the impending parents started to change their mind. Instead, they keep their committment and as Oz is leaving, Geordie gives him a new soccer ball. The scene was bittersweet as it was clear Geordie had a connection to Oz and could envision adopting Oz himself.
While Oz receives the soccer ball, Nurse Butcher gives Alphy the doll from earlier in the episode and explains it was Alphy's when he was a young boy at the orphanage.
Before Oz left, Stuart had given him and his new parents a box that was left by Oz's birth parents. Alphy asks Stuart if he also has something that was left for him. Stuart gives Alphy an envelope with a note, which Alphy takes home and throws in a drawer.
In other parts of the episode, Geordie's wife, Kathy, tells the rectory's caretaker, Mrs. Chapman, that she has left her job. The two decide to go into business together.
Curate Leonard shows up at the rectory but Alphy is worried he is constantly hungover. We learn Leonard obviously has a growing alcohol dependence.
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