Season 04 Episode 03

Geordie is being fitted for a suit at the department store where Cathy now works, attended by a colleague of hers, Anthony Hobbs. Leonard takes the Sunday address in Sidney's absence, and Jack Chapman writes a letter recommending him for the vacant parish.

Geordie is called to a reported break-in at a mathematics laboratory that houses UNIAC, a room-sized computing machine. Alex Simms, who runs the laboratory, answers the door and tries to turn him away; Tim Amery, his young colleague, had called it in, but the supposed intruder was Simms, who had spent the night there. The other two members of the team are Simms's wife Jean and the laboratory's mechanic, Karla Read. Amery talks up a coming generation of smaller computers while Simms behaves erratically and smells of drink. He vomits, collapses and dies. A broken tube is leaking mercury, and the room is cleared.

Geordie learns that Simms was once arrested on a homosexual offence and released without charge, which took friends in high places, and finds a book inscribed to Alex and signed B in the laboratory. He turns to Leonard for help, partly because Leonard knows where men meet in private, and Leonard and Daniel direct him to the Ganymede Club. Geordie and Leonard get in, and Leonard signs the book as Sidney Chambers, which lets Geordie match the inscription to a recent entry in the name of Robert Hazelwood. Jean Simms is in the club, and runs when she sees Geordie. Brought back, Jean explains the arrangement: she married Alex to keep his name clear, and his lover of about a year was Bobby Guthrie, whose pen name is Robert Hazelwood and who was with Alex in the laboratory that night. Bobby is now ill at home with mercury poisoning, and Geordie finds tape from the machine in his jacket and tells his wife about the affair. Leonard, who has watched Geordie out a man who is already frightened and ill, tells him he is finished with the case.

Geordie finds Jean's wedding ring near the broken tube and concludes that the jealous wife killed her husband, but Karla Read says that Jean could not have, because she left with Karla that night and Karla and Jean are lovers, so there was nothing for Jean to be jealous about. Amery then reads the tape taken from Bobby's jacket: it is an image of a heart and the phrase two bodies, one soul, Aristotle's description of love, which Bobby has copied. Will Davenport and Leonard push Geordie to look again. Geordie returns to the laboratory and finds what he needs. Tim Amery called in the false break-in to get the police into the laboratory, broke the mercury tube himself, and left Jean's ring near it so the police would settle for the wife. He wanted Alex, whom he believed washed up, out of the way so they could write again on newer machines. Amery smashes UNIAC with a wrench when Geordie confronts him and is knocked down and arrested.

Elsewhere, Mrs Chapman, having caught Leonard and Daniel, will not speak for him before the archdeacon, and Leonard tells the archdeacon he is not ready. Will is given the parish of Grantchester instead, arriving on a motorcycle and shaking the archdeacon's hand. Cathy goes back to work and Anthony Hobbs corners her in a room and threatens her, and she takes her complaint to her manager and is effectively told that the misunderstanding is over. Will Davenport takes over the parish from this episode, and it is the end of Sidney Chambers in the role.

The episode is the most interesting of Series 4 because the murder is committed by a colleague who wanted a machine replaced, the police's first answer is the wife, and the tape is what undoes that reading. The motive is unusually modern for the series, with a young man talking about smaller computers while killing for the chance to build one, and the arrangement between Jean, Alex and Bobby is written without apology. Leonard turning his back on the case, and the manager's response to Cathy's complaint, are the two points where the episode says what it thinks about 1956.