During the 1940 Battle of France, Rosemary Brown [Patricia Roc], an English novice nun, is apprehended by French soldiers who have mistaken her for a fifth columnist. She is sentenced to face a firing squad, but the Germans arrive and she is sent (without her habit, which is being cleaned) to an internment camp in a grand hotel at the spa town of Marneville. She travels there in the back of a lorry with journalist Freda [Phyllis Calvert], stripper Bridie [Jean Kent], and posh Muriel [Flora Robson] and her travelling companion, Miss Meredith [Muriel Aked].
At the camp, they meet Maud [Renée Houston], Margaret [Anne Crawford], Nellie [Dulcie Gray], Mrs Burtshaw [Thora Hird] and Teresa King [Betty Jardine]. Two women are allocated to each room, but Bridie uses her charms with Sergeant Hentzner [Carl Jaffe] to obtain a room to herself. Although the hotel is very luxurious, not all the baths have a water supply, and the proprietor, Monsieur Boper [Guy Le Feuvre], is believed to be collaborating with the Germans.
The women receive a radio from an unknown source, but it is swiftly confiscated by the Germans. They suspect there is a stool pigeon among the dozen who knew about the radio, nicknamed “Poison Ivy.” Nellie reports that she saw the German file on Rosemary; the charge of being a fifth columnist casts suspicion on her. However, Freda and Maud do not believe it. They warn Rosemary, who reveals she is a nun.
Freda deliberately violates the blackout during a night-time air raid by the RAF. One plane crashes nearby after its crew bail out. Pilot Officer Jimmy Moore [James McKechnie], Sergeant Alec Harvey [Reginald Purdell] and Dave Kennedy [Robert Arden] seek refuge in the hotel. The women hide them, but have to conceal the fact from Teresa King, who is revealed to be a Nazi spy. Later, Alec recognises Rosemary as Mary Maugham, a singer whose boyfriend murdered his wife; she became a nun as a result. Jimmy and Rosemary begin to fall for each other, as do Dave and Bridie. Hentzner finds Dave, who manages to strangle him quietly, and his body is hidden.
The women devise a plan to enable the men to escape during a concert they will put on. To ensure the Germans stay until the end, Freda persuades Bridie to perform her act last. However, when Bridie overhears what Dave thinks of her (due to her fraternisation with the Germans), she slips Teresa a note betraying all. Freda makes Dave write an apology professing his love, which she delivers to Bridie. Bridie then goes to Teresa's room and sees that she has already read the note. The two women fight. Teresa wins and alerts Frau Holweg [Christina Forbes], but Maud knocks Holweg out. However, Teresa sees the airmen escaping and warns the commandant, but it is too late. The trio escape, with the aid of Monsieur Boper, who is not a collaborator after all. The women defy the occupiers and sing “There’ll Always Be an England.”
