An exploding bomb sends Lt. George to the Field Hospital. Blackadder ends up at the hospital too, but for different reasons. Gen. Melchett has set up a spy ring upon hearing that the hospital is the source of a security leak. He gives Blackadder three weeks to find the German spy. At the same time, Melchett sends Captain Darling to the hospital to spy on Blackadder. No way is Melchett going to have the enemy learn of yet another British movement. Blackadder interrogates Darling, attracting the attention of George's nurse, Mary Fletcher-Brown. She spends a lot of her intermittent time with Blackadder while a man with a German accent keeps a watch on George. After three weeks, Blackadder reports to Staff HQ. He reveals that an intimate moment with the Nurse was a probe in more ways than one. From that, Blackadder says, Nurse Fletcher-Brown is the German spy. Darling discovers to his horror that the man with the German accent really works for British intelligence. George is shocked at all this, but when he reveals that he has been pouring out news from the front in letters to his German uncle, Blackadder is the most shocked of all.