In the year before WWII, 165 Eaton Place reopens its doors and welcomes viewers back into the lives of its inhabitants, upstairs and down. Lady Agnes and Sir Hallam’s family is now complete with the addition of two small children, but Sir Hallam is drawn into dangerous waters by his preoccupation with the threat of Nazi Germany and the return of troubled Lady Persie. Lady Agnes catches the eye of charismatic American multi-millionaire Casper Landry, and Sir Hallam’s aunt, Dr. Blanche Mottershead, has made herself comfortable in the house after Lady Maud’s death. Meanwhile, life downstairs adjusts to the absence of Rose and the addition of the spirited new nursery maid, Beryl. With upstairs and downstairs harboring life-changing secrets, and the menace of war creeping ever closer, the smooth running of Eaton Place threatens to come to a halt.
Gas masks are issued and long-buried secrets about the staff are revealed against an increasingly uneasy political backdrop. While the house staff struggles with the juxtaposition of patriotism and conscience, Sir Hallam stands firm in his diplomatic convictions, helping to negotiate what may be the final chance to avert war. But as the possibility of maintaining peace becomes more apparent, will England be able to sleep quietly for long?