The Halcyon's owner Lord Hamilton holds a secret meeting that attracts the interest of American guest Joe O'Hara, while manager Richard Garland must deal with the arrival of Lady Hamilton a day earlier than expected.

Lady Hamilton wants to make changes and Garland is in her sights. The hotel manager insists he's not worried and that he just needs to show how indispensable he is, but Peggy and Feldman aren't so sure. Charity meets Joe and reveals her plans for revenge, but he refuses to help her. Emma and Betsey hit the town.

In the wake of the fall of France to the Germans, guests flood into the hotel, including one who catches Lady Hamilton's eye - the Comte de St Claire. Peggy worries about Billy's enthusiasm to sign up as soon as he turns 18, but knows there is little she can do to protect him. When Max faces hostility from the staff, Emma goes out on a limb to defend him - and immediately regrets it.

Joe is offered the opportunity of a lifetime, but finds himself unexpectedly torn as the moral responsibility of staying in England to tell the story of the war weighs heavily on his shoulders. Betsey's mother comes for a surprise stay, leaving Sonny unsettled and wary of the reaction she incites from Betsey, while Lucian and Lady Hamilton continue their flirtation and Emma anxiously awaits news of Freddie when she hears that a plane from Squadron 392 has gone down.

Emma is in charge of the hotel and is put to the test hosting a society wedding for Lady Hamilton's friends the Ashworths. Celina Radimsky comes to the hotel to meet Freddie, who tries to offer consolation after her husband's death. Garland goes to the East End to help Peggy with some housekeeping, but has an encounter with a figure from his past while waiting outside a shop.

Garland asks Emma to take over running the celebrations for the 50th anniversary of the hotel, and Betsey catches the eye of a handsome man in the bar. Meanwhile MI5 officers arrive with a warrant to search the premises.

The Halcyon celebrates its 50th anniversary. Lucian struggles to keep things in order as the pressure mounts. Garland and Lady Hamilton find a cause together.