This is one of a series of first-rate British movies Frank Launder and Sidney Gilliat wrote and, in a number of cases, directed starting in the 1930s.
Deborah Kerr plays Bridie Quilty, a young Irish woman who was brought up to despise the British. Its 1944 and Ireland has stayed neutral in WWII. When she reaches her majority she is determined to join the IRA and fight against the Brits. She travels to Dublin to seek out the IRA and is rebuffed, but is recruited by, unknown to her, a German spy. Raymond Huntley, a great English character actor, plays the spy. He has her finding out information as a worker in a pub, next to a British army base just across the border. Unexpectedly, she meets a young Army offficer (Trevor Howard) who is in counter-intelligence, and then comes across a great secret which, she is told, must be delivered to an agent she thinks is fighting against the Brits on behalf of the Irish, but is actually a sleeper Nazi. Bridie's adventures are many, some romantic (although she can't stand the idea of falling for a British officer), some funny, some dangerous. The conclusion, where if Bridie is caught on the Northern Ireland side of the border she'll be hanged, but if she can cross the border to Ireland she'll be safe, is a nice little drama of its own. It causes a quandry of conscience for Howard, and is resolved neatly.
This is a charming and expertly made movie. Deborah Kerr, at 24, brings glowing naivete to the part. After Kerr made this and Black Narcissus (1947), she was off to the USA.
Launder and Gilliat's films read like a roster of quality and craftsmanship. Among them are The Lady Vanishes, Night Train to Munich, The Rake's Progress, Green for Danger, The Belles of St. Trinian's, The Green Man, Geordie and Young Mr. Pitt. Except for The Lady Vanishes, none are out on DVD in the U.S. and should be.