Contains spoilers for this episode.
A bomb interrupts the trench's guessing games and sends George to hospital, where Nurse Mary Fletcher-Brown helps him write letters to his relatives. Melchett believes that a spy is leaking battle plans from the hospital and gives Blackadder three weeks to find the culprit. Success promises command of a new intelligence operation. Darling wants to supervise the investigation, so the general shoots him in the foot to provide a reason for his admission.
Blackadder subjects Darling to an insulting interrogation and orders Baldrick to watch a patient called Smith, whose strong German accent makes him conspicuous. Mary proves much less sentimental than her bedside manner suggests, and she and Edmund begin a relationship. At headquarters, Blackadder eventually identifies her as the spy, citing her German vocabulary, questions about tanks and failure to recognise his joke about supposedly great universities. Baldrick takes her away while Melchett prepares a firing squad.
Darling brings in Smith as his preferred suspect, only to learn that the patient is a distinguished British operative whose years undercover explain his accent. Melchett rewards Blackadder with the new appointment. George then supplies the information the investigation has missed: his letters have been going to Uncle Hermann in Munich, complete with military news and remarks about Melchett's dirty shirts. Darling offers Blackadder the uncomfortable task of explaining this revelation to the general, puncturing his apparent intelligence triumph.