Bridie Quilty has grown up enthralled by her father's Irish rebel past, and decides to go to Dublin to follow in his footsteps. She shares a train compartment with an Englishman, Miller, who is amused by her romanticised view of relations between Britain and Ireland.
In Dublin, she visits Michael O'Callaghan, a former rebel turned respectable gallery director. She says she wants to join the IRA - but he now prefers negotiation to armed conflict.
In a bookshop, Miller (a German spy) meets a man who asks him to spring fellow spy Oscar Pryce from prison, as he has information vital to their cause. Miller bumps into Bridie, and realises she could be an ideal collaborator.
In the English town of Wynbridge Vale, a statue of Oliver Cromwell is vandalised. Bridie has started working at the local inn, the George, where she is unexpectedly friendly with English soldiers. She's actually helping Miller obtain information on Pryce's movements.
Miller asks her to spend an afternoon diverting David Bayne, a British intelligence officer. Miller helps Pryce escape, but both become fatally wounded - Pryce telling Miller as he dies that his secrets are hidden on the Isle of Man.
Miller returns to the George to tell Bridie that she needs to pass on Pryce's information to a fellow spy, who will be in a particular train compartment the next day. He dies, and Bridie disguises the body as a wheelchair-bound old man, pushing it through the town and tipping it over a cliff.
Bridie finds the train compartment, but no-one reacts when she gives the requested signal. When the train pulls into the station, the old lady sitting next to her is arrested. Bridie bumps into Bayne, who's been following her. While he's making small talk, she sees a newspaper headline about German spies in Wynbridge. A travel ban to Ireland scuppers her original plans, so she carries out the Isle of Man mission instead.
Unknown to her, she's being trailed both by one of Miller's associates, a man in a straw boater, and by the police, who found Miller's body and put two and two together. They alert their Manx colleagues, who are somewhat lackadaisical.
Bridie goes to a courtroom and finds a diary hidden in a public gallery seat. It contains details of the D-Day landings, and Bridie realises that if the Germans find out where they're to be held, this will have disastrous consequences for the troops - not just British, but Irish too. Thousands of lives are in her hands.
She returns to her hotel room, and burns the diary. The police come to check her ID. She has a fake one in the name of Mrs David Bayne - and her "husband" turns up at this point. He plays along, but when the police have gone he asks her what's going on - and confesses that he's fallen in love with her. She tells him everything, and he says he'll have to turn her in.
When the maid comes to make the bed, Bridie grabs the opportunity to flee - but she's then kidnapped by the man in the straw boater. Bayne goes out to the harbour to investigate a freshly-moored boat, and is captured as well.
The boat sails to Ireland, and the spies bundle Bridie and Bayne into a horse-drawn carriage that inadvertently ends up as part of a funeral procession - providing perfect cover until the police expose them as smugglers. In the confusion, Bridie and Bayne escape to a nearby inn.
Bayne offers to turn Bridie into the Irish police, who will merely intern her, whereas the British would have her shot. Unfortunately, he only realises that they're actually in Northern Ireland after the landlord has summoned the Ulster police. A radio announces the D-Day landings, providing cover for Bayne to help Bridie flee through an upstairs window. He then discovers that he's stumbled upon the spies' hideout, and a fight ensues - eventually broken up by the police.
At the end of the war, Bridie and Bayne are married - but their honeymoon gets off to a rocky start when she refuses to spend the night in the Cromwell Arms.
