Season 05 Episode 04

A young man runs naked through Grantchester in daylight shouting that the aliens are coming. He collides with Geordie and Will, and Will persuades Geordie to let it go as high spirits. He is later found dead in a field on the Fens, still naked, with no alcohol in his blood: Chris Hartley, a psychology undergraduate at Trinity Hall. Ellie Harding has his name in the paper before the police do and trades it to Will for dinner.

Chris's room has been searched. He kept a journal, and the entry ML leads to the Master's Lodge, where Douglas and Judy Munroe receive the news. Judy is a Fellow in Psychology at Girton who had been advising Chris, and he was due to see her the next day. Douglas mentions the trial into the therapeutic use of LSD, run by Martin Adler, in which Chris was a participant. Chris's father identifies the body. Adler is unapologetic. He says the Munroes resented his funding, that LSD is not a route to madness but God in a bottle, and that he keeps it locked up. Remi Adeyemi, a participant, speaks warmly of the experience and says it gave him his old self back. At the laboratory Will and Geordie meet Peter Soosai, who admits he synthesised extra LSD and gave it to Remi, who took it intending to share it with Chris at the weekend. When Remi heard Chris had been running naked he looked under his bed and found the phial gone.

Will, working through Chris's annotated copy of Aldous Huxley's The Doors of Perception, cannot square the size of the dose with a timid student. His girlfriend, Helen Feldman, says she talked him into the trial, that he was frightened of taking LSD outside it, and that it has no taste. Will then speaks to Judy Munroe, who tries to dissuade him from taking a dose himself; he tells her he feels responsible, because he and Geordie saw Chris and did nothing. She tells him Chris's mother left when he was small and a recent visit home left him badly upset. Adler shows Will a film of Chris's session, in which Chris recalls his father choking his mother and says his father killed her. Confronted, Mr Hartley says his wife was having an affair with a Canadian airman and was leaving, that he tried to stop her and it turned physical, and that he did not kill her. He had told his son she went to Canada when she had gone to Bedford, and he came to Grantchester to tell him the truth.

Will works from a domestic argument rather than a clue. Mrs Chapman's discovery about Jack Chapman's money, and the question of what a woman does with a fury she cannot express, lead him back to Judy Munroe, whom he accuses of putting the LSD in Chris's tea. She does not deny it, and says she did not know it would hit him so hard, that Will should have stopped him, and that Adler should never have been allowed to run the research. She is taken away.

Elsewhere, the Keatings' dinner party collapses when Diana nearly burns the house down. Cathy tells Geordie that her mother's depressive episodes last months and that she was aggressive and abusive to her as a child. Geordie arranges for Diana to be taken to hospital against Cathy's wishes and tells her himself before the ambulance leaves. Will asks Ellie to marry him; she laughs, says that is not the answer and that they barely know each other, and refuses. Leonard, having failed to lift Mrs Chapman out of the discovery that her marriage rests on a lie, writes to his father and invites him to stay.

The episode is the strongest of Series 5 and one of the best in the run, because the solution comes through the vicar's understanding of a marriage rather than through police work, and the man the film appears to convict is cleared properly. The LSD trial is handled as an argument between people who all have a case, and the drug's lack of taste is what makes the tea the answer. Judy Munroe telling Will he should have stopped Chris is the accusation he has already made to himself.