The episode starts immediately after the end of the previous one. Lady Agnes asks the servants to look after the drunk Mr Pritchard. She narrowly misses catching her husband and her sister in flagrante delicto. When she makes breakfast herself, she notices that Harry and Beryl show up together. Unaware of the marriage plans of Harry and Beryl, she accuses Beryl of immoral behaviour. Beryl tells her that she's the last person to talk about these things. If only she knew what's been going on in 165 Eaton Place.
Suspecting that something strange has been going on during her absence, Lady Agnes wants to have her family together again. She telephones Blanche and asks her to come home. Meanwhile Hallam mentions his suspicions about Persie to his superiors at the Foreign Office. Persie wants to leave London, but Lady Agnes feels she should stay.
Harry and Johnny get their call-up papers. Harry assures Beryl that they will leave for America before he has to go for his medical. They only need a marriage licence. That turns out to be more difficult than expected.
Pritchard believes his career as a butler is over after his drunken misbehaviour. Johnny expresses his disappointment in the man he saw as a role model. Lady Agnes has a talk with Eunice, hoping to find more about the previous weeks. Eunice doesn't really say much, but she fears she may have revealed too much.
With the start of the war expected any day, Mrs Thackeray starts hoarding food. She asks Mr Pritchard to get some treacle for a fruitcake. Pritchard leaves and isn't seen for days. Is he on a bender? Preparations for war continue as sandbags are put around the house and blackout measures are taken. Hallam asks Mr Amanjit to clean his pistol.
Johnny goes to visit Rose at the sanatarium to talk about Pritchard. She explains that Pritchard was fired from the cruise liner for being drunk. Johnny goes looking for Mr Pritchard among the drunks along the embankment.
Persie calls her German friend Friedrich for help, but he says that they are now two different people. Lady Agnes helps the servants unpack the laundry. She finds a note mentioning a lipstick stain on Hallam's shirt. She immediately checks Hallam's clothes for further signs of an affair. Caspar Landry drops by with some nylons. He's leaving for America and feels Lady Agnes could use some stocking if the war lasts a long time.
Hallam gets an official report that Persie has been seen with Germans in London. She will be arrested and imprisoned in the next few days. Lady Persie, Blanche and Persie have tea and discuss lipstick. Persie mentions that she uses a type that leaves marks that can't be removed. This arouses Lady Agnes's suspicions, and she searches Persie's room. The lipstick matches the stain on Hallam's shirt. She can also smell Hallam's cologne on Persie's sheets. Once the secret has come out, Persie and Agnes get into a dreadful fight. The servants are quite shocked, but not surprised.
After it came out that his sister-in-law will be arrested for treason, Hallam resigns from his post at the Foreign Office. He goes for a walk in the park and bumps into Pritchard. Then he tells Lady Agnes what he has done. After all, he was betrayed by Persie. Lady Agnes assures him she knows the feeling.
Persie realises that her days of freedom are numbered and she begs Hallam for help. He ignores her. Meanwhile Harry finally gets a marriage licence. Looking for Beryl, he bumps into Persie, who sitting in the drawing room holding Hallam's pistol. As Harry tries to pry the gun from Persie, it goes off ... and hits Beryl who walks into the room. Persie is so upset that she jumps off the balcony and kills herself.
Pritchard sees a body being taken away from 165 Eaton Place and returns to duty. He finally brings Mrs Thackeray her tin of treacle. War is announced on the radio.
After Persie's funeral Lady Agnes confesses to Hallam that she feels guilty for Persie's death. Persie was never loved enough. She tells her husband that she will send the children to the countryside for the duration of the war. Rose will stay with them. Divorce is out of the question, considering the circumstances.
The servants celebrate the wedding of Harry and (a slightly injured) Beryl. Hallam gets an offer from George, Duke of Kent: he can be the Duke equirry during the war. It means a job at Buckingham Palace. Harry returns the money he took from Hallam. He explains that America is out of the question for the moment. Due to Beryl's injury they missed the boat.
The first air raid alarm is heard. Everybody goes downstairs, except Pritchard, who starts his duties as air raid warden.