With no sign of a thaw in relations between Lady Agnes and Maud, Hallam finds his diplomatic skills put to the test at home as well as at work. The servants are intrigued by a mysterious new addition to the household, while a restless Lady Persie embarks on a dangerous flirtation, and is drawn to the thrill of radical politics.

Sir Hallam finds himself at the centre of a national crisis when his friend, the Duke of Kent, asks for help. Meanwhile, storm clouds are gathering within 165 Eaton Place, with Lady Agnes's growing fear over the imminent arrival of her baby, and the servants' concern for Lotte, who refuses to speak. But, as the king finally abdicates, Hallam makes a discovery that will change his life forever.

Diplomat Sir Hallam Holland moves into the iconic townhouse along with his wife, Lady Agnes. With the help of former parlormaid Rose Buck, they launch a whole new era for the sumptuous home at the heart of Upstairs Downstairs.