Season 11 Episode 04

Alphy's car tyres have been slashed and he takes the train to see his mother instead of driving. At Mira's house a young man, Rajesh Kohli, answers the door. He says his mother is out, mentions a father, mocks Alphy's clerical clothes and does not recognise Alphy's name. Alphy works out that Mira has a second family she has never mentioned and that she has told them nothing about him, and he leaves without explaining himself.

Mira finds him in a park nearby. He tells her he could forgive the second family and that what he cannot forgive is the lie. She defends herself by saying she does not understand why he is a priest and finds it embarrassing, and he replies that she is the one who left him at a Christian orphanage. He throws the flowers he brought into a bin and goes back to the station. In Grantchester, Larry brings in Raymond Hayes again, and Raymond admits to all of it, including slashing Alphy's tyres. Leonard tries to defend him on the grounds that his mother is dead and his father Marcus is ill, and Geordie argues that the whole Hayes family has been through the courts and that the state may be the best place for the boy. Leonard, on Larry's advice, persuades all of Raymond's complainants to withdraw their allegations on the grounds that juvenile court would not help the community, which annoys Geordie and enrages Raymond, who throws a tantrum at Leonard for interfering. It emerges later that Raymond wanted to go to juvenile court because he believes his sick father would be better off without him to care for.

On the train home Alphy is woken to comfort a crying woman. The train has broken down. The woman, Flora Crabtree, is sitting beside her sleeping friend, and Alphy realises the friend, Matilda Garland, known as Matty, is dead. He prays for her, notices blood on her fingers, and finds a piece of metal embedded in her arm. A passenger called Jeff Stanwick says he heard Matty speaking after the previous stop, so the train's employee, Birde, locks everyone in the carriage, since nobody else has passed through and one of them killed her. The passengers are Alphy, Flora, Jeff, Amelda Frow, a young woman who had been talking to Jeff and who offers advice because her father was a doctor, and Clarence Walmer, a fare dodger who tried to run before anyone knew Matty was dead. Alphy had overheard Flora and Matty discussing something in a newspaper that upset Matty, and Flora explains that an anonymous letter had questioned Matty's charitable work. Matty was wealthy, as her expensive necklace showed, and her newspaper has Ridley written on it in her own hand. Flora says Matty was her best friend, and difficult, and that Matty went to get tea after the breakdown and came back and slept.

Amelda notices the necklace is missing, and Alphy notices Amelda's wedding ring is missing too, from the tan line, and works out that she is having an affair with Jeff. Watching Flora eating chocolate, Amelda guesses she is diabetic, and Alphy realises she would carry a needle for insulin, which could have been used to stab Matty and inject air to cause a fatal embolism. He persuades Birde to let him search the luggage compartment, finds insulin in Flora's bag but no syringe, goes outside and finds the syringe in the grass by the gangway door. Flora denies the killing but admits she wrote the anonymous letter, because she wanted to take Matty down a peg. The passengers empty their pockets and Matty's necklace is found in Alphy's, and Jeff takes the opportunity to treat his racism as vindicated and uses a pair of handcuffs from the lost-property box to restrain Alphy. Alphy then notices a velvet pouch in the same pocket and recognises it from Birde's office, and accuses Birde of the theft, and Birde slaps him. Clarence, who had said the route was known for pickpockets, demands to see Birde's office and finds a second velvet pouch in Birde's pocket. Birde admits stealing from passengers to supplement his pay and denies the murder, complaining that the railway bosses want to cut half the lines and the jobs.

The killer is Amelda. As she treats the wound Birde gave Alphy, she mentions her father, Dr Ridley, and Alphy connects the name to the writing on Matty's newspaper. Matty knew Amelda and recognised that she was having an affair, and Amelda begged her to keep quiet and then killed her to keep her silent. She had noticed Flora was diabetic, took her needle, stabbed Matty as she went to get tea, and threw the syringe out of the door. The affair is the escape she cannot lose. Amelda and Birde are arrested when the train reaches Grantchester. Geordie and Leonard then revise their view of Raymond, having understood that he wanted the court to take him so that his father would not have to look after him. Miss Scott, meanwhile, has been waiting on test results and has told Geordie she has not yet told Larry, who is waiting on the result of his detective examination, and Larry learns at the end of the episode that he has been promoted to sergeant, and Miss Scott receives confirmation that she is pregnant.

The episode is a closed-circle puzzle with six people in a locked carriage, and the weapon is a piece of medical equipment borrowed from one passenger by another, which is the neatest construction of the series. The tan line where a wedding ring should be is the detail that opens the affair, and the name written on a newspaper is what closes it, since the doctor's daughter is named after her father. Raymond's tantrum is the other half of the episode, because the boy the adults were arguing over was trying to get himself taken away to protect his father.