Rachel James (Julia Dalkin), a physiotherapist, is shot through a window of Number 11 Bloxham Drive, with the shade drawn as she is eating breakfast between 7:00 and 7:30 AM on Friday morning. An unsigned valentine is discovered in her belongings. Her neighbors include Geoffrey Owens (Mark McGann), a local reporter, and Adele Cecil (Judy Loe).
Dr. Julian Storrs (John Shrapnel) and Denis Cornford (Roger Allam) are the two candidates in an intense rivalry for Master of Londsdale College to replace Sir Clixby Bream (Richard Briers).
Detective Chief Inspector Endeavour Morse and Detective Sergeant Robert Lewis begin the investigation with the neighbours and the clinic where Rachel worked. Morse soon establishes that Julian Storrs gave Rachel the valentine card and was having an affair with her. Morse also learns from Storrs that Denis Cornfeld and Adele were once lovers.
Geoffrey Owens is found shot dead in his flat at Number 15 Bloxham Drive, which is next door to Rachel's between 7:00 and 7:30 AM on Saturday, the following morning. Because there is no Number 13, Morse concludes that Rachel James was mistakenly killed and Geoffrey Owens was the intended victim. Morse also finds out that Owens supplemented his reporters income by blackmailing unknown victims. Among Owens' papers Morse finds a slip of paper in a file with an article he had written about the retirement of Bream with the text, "AM→DC→CB" on it. He also finds a case where Alice Martin and her daughter (Debra) shot a wealthy businessman, Kenneth Martin, and then burned him on his yacht because he was going to run off with a younger woman. Lewis suggests that Alice Martin is "AM".
Bream intimidates Shelley Cornfeld (Holley Chant), the wife of Denis, into having a sex with him in exchange for supporting her husband Denis for the master position. After he has finished Bream then tells Shelley that he will not support Denis and he has no chance for the job of the Master, and he had sex with her in revenge for Denis having had an affair with Clixby's wife years before.
Morse finds out that Alice Martin and her daughter Debra changed their names to Angela and Diane Cullingham, to avoid the stain of their past following them. And then Ms. Angela Cullingham married Dr. Julian Storrs.
When Shelly Cornfeld tells her husband what she had done with Clixby Bream it leads to a violent confrontation in which Shelley is accidentally killed falling down a flight of steps at college.
Lewis suggests that "DC" is Diane Cullingham and that leads Morse and Lewis to Bath to question Dr. Julian and Angela Storrs (Maggie Stead) who are there for a conference.
While Morse and Lewis are questioning the hotel staff and then the Storrs, Diane Cullingham (Nicola Jeffries), the daughter of Angela, is also questioned and detained at her home by Detective Constable Jackson. Diane Cullingham also stayed at the hotel in Bath on Friday night.
Angela Storrs killed Owens because he was blackmailing her over the death of her first husband years early under a different name, "Alice Martin", and running an expose now would ruin her husband's chances of becoming Master. She killed Rachel James by mistake and went back the next morning to try again.
Morse threatens to expose Bream for his role in the death of Shelley unless he gives up his continuance of Master and leaves Oxford.
At the end of this episode Morse reveals that his first name is "Endeavour", after Captain Cook's ship, HMS Endeavour.