George ends up in the field hospital where he meets soppy Nurse Mary and reads a letter to her. Blackadder and Baldrick enters the hospital, where Blackadder insults Nurse Mary, accuses George of being a "malingering git" and demands the food sent by George's relatives, only to find it has all been eaten. Baldrick presents George with a bunch of dead stalks (that have had their heads shot off). Blackadder is summoned by General Melchett to HQ after offending Nurse Mary and meeting a suspicious German speaking man, called Mr Smith, as he leaves the hospital.
Blackadder enters the HQ, sees Melchett's face behind a map covered board and is immediately frisked by Darling, who gives the all-clear. Blackadder wants to know what is going on and is told by Melchett that there is something sinister and grotesque and it's right under his nose (and surprisingly enough it's not the general's lovely moustache). There appears to be a German spy about, who is giving away British battle plans (such as moving to Boulogne, only to find the Germans have bought all the toilet paper). This surprises Blackadder, who didn't realise there were any battle plans or that battles were directed, until he is informed that there is a grand plan (which he straight away names as the plan to "continue with total slaughter until everybody's dead except Field Marshal Haig, Lady Haig and their tortoise Alan", which causes great alarm in General Melchett as the security leak is worse than he'd imagined).
Darling informs Blackadder that Haig is anxious to eliminate all German spies ("filthy Hun weasels fighting a dirty underhanded war", says Melchett) and that one of the British spies ("brave heroes risking life and limb for Blighty", says Melchett) has discovered that the leak comes from the field hospital. Blackadder is charged with finding the spy. He asks for 6 months to do so but is given 3 weeks. Melchett recommends getting a cocker spaniel, tying the suspect to a chair, putting a potty on his head, popping his todger between two floury baps and shouting "Dinner time Fido!". If he succeeds, Blackadder will be made Head of Operation Winkle ("to winkle out the spies").
Blackadder and Baldrick return to the hospital, interrupting George's letter reading and again upsetting the nurse. George is informed of the German spy and promptly comes up with a way to find him, which is to look at the list of patients and see if anyone has "von" in their name. He finds the idea of the spy using a fake name unfair. Baldrick suggests asking everyone "Are you a German spy?" and says he would own up. Blackadder points out that the Germans have not yet added the requirement "must have intellectual capacity of a boiled potato" to their army entrance form. He then assigns Baldrick to watch the suspicious Mr Smith.
Darling enters, limping with a cane. Having suggested to Melchett that someone should keep an eye on Blackadder ("Spy on our spy trying to find their spy, you men?"), Darling has been shot in the foot by Melchett in order to have "a convincing wound" requiring hospital treatment. Blackadder of course takes advantage of this opportunity to humiliate his rival by suggesting that the General was aiming for his head. When Darling insists that he "wasn't aiming for anything", Blackadder replies, "Oh, so he was going for between your legs, then." Blackadder then informs Darling (who snarls Blackadder will 'be laughing on the other side of your face if you don't find the spy!') that he intends to interview suspects immediately.
We then see Darling in the position Melchett suggested to Blackadder earlier. Blackadder asks a few questions about the Queen of England and the Kaiser, enjoying his power over Darling. His mention of the cocker spaniel interrogation method mentioned by Melchett frightens Darling, who insists he is as British as Queen Victoria ("You mean your father's German, you're half German and you married a German" says Blackadder).
The loudness of the "interrogation" brings Nurse Mary in to complain. Darling leaves, still attached to the chair and swearing revenge (he doesn't get far before seemingly injuring himself) . Left alone with Blackadder for the first time, Mary becomes more sincere, and reveals that her earlier behaviour was her "fluffy bunny act" and offers him a cigarette. Blackadder replies that he only smokes after sex, which makes him "a 20-a-day man" back in England. The two talk, when Mary suggests they get to know each other better. Blackadder suggests a walk in the country, while Mary simply suggests 'we could just do it right now on the desk!'. Blackadder swiftly agrees, and the pair end up having sex on the desk.
While George asks Smith if he's seen any potential German spies (he responds "Nein", but George thinks the man said "Nine"), Blackadder and Mary discuss special people in their lives (the only one in Blackadder's is himself), tanks and great English universities. Blackadder informs Mary that his time to find the spy is up and that he is going back to Melchett's HQ. He invites her to come with him.
Blackadder and Mary arrive at HQ and talk to Melchett, who has received a letter from the Germans calling him Walrus Face and asking when he is going to change his shirt. Mary voices a suspicion that the spy may be Darling as he had "pooh-poohed" Blackadder. This causes Melchett to talk about how ignoring a pooh-pooh is a fatal error and that a regiment once had to be disbanded as its morale was destroyed by pooh-pooh. Blackadder dismisses Mary's hypothesis saying that Darling is unfortunately not the spy. Mary then says it must be the German speaking man. Blackadder points out that even the Germans wouldn't be stupid enough to field a spy with a strong German accent. He then accuses Mary of being the spy, giving three pieces of evidence. First, she speaks German. Second, she asked about tank movements. Third, she failed to spot that of the three universities Blackadder mentioned in their earlier conversation, only two could be considered great (Oxford, Cambridge and Hull--although there is now a University of Hull, there was not at the time). Melchett (a Cambridge man) agrees, as "Oxford's a complete dump". Mary is taken away to a firing squad.
As Melchett is about to use the phone, Smith approaches, followed by Darling, who leaps on Smith, thinking he is trying to kill Melchett. After a brief scuffle between the two, Darling pulls a gun and asks if Smith is a spy, who answers that he is. Melchett says that the man is Brigadier Sir Bernard Proudfoot-Smith, the best spy the British have. Darling points out the German accent and Smith says it is because he has been undercover in Germany so long. Melchett informs a chastened Darling that Smith was the one who warned of the spy in the hospital originally.
Blackadder is made Head of Operation Winkle, while Darling is told that he is a complete arse. Melchett and Smith leave to watch the firing squad as George enters and blithely reveals that he has been writing letters to his Uncle Herman in Munich, telling him about "Walrus Face" Melchett and his smelly shirts. Blackadder and Darling realise that George is the leak and Darling smugly asks Blackadder who he would like to inform Melchett of this. They dash out of the room, past an utterly confused George, as the episode ends.
Note: Nurse Mary's fate is unknown. According to rumour, the message to alert the firing squad she wasn't the spy was delegated to Baldrick.