Ireland, 1937. Bridie Quilty [Deborah Kerr] grows up listening to her father's heroic tales of the Irish Revolution, cultivating a deep-seated hatred for Britain and, in particular, Cromwell. By 1944, with her father dead, Bridie, now 21, travels to Dublin to forge her own life and to pursue a path that could lead to the IRA. On a train to the city, she shares a compartment with J. Miller [Raymond Huntley], a businessman returning from abroad who is secretly a German agent, slipped into Ireland through lax security. His assignment becomes clear: to break a fellow spy out of prison.
Miller recruits Bridie, who quickly finds work at a hotel and bar in Wynbridge Vale, where she begins to learn the tricks of distraction and disguise. She also grows close to a certain sergeant who unwittingly supplies information about the prison transfer to London. On the day of the transfer, Miller tells Bridie to keep Lt. David Bayne [Trevor Howard] distracted—an adult naval officer on leave who is researching Cromwell and may be a counter-intelligence risk.
The spy is freed but is shot during the getaway. Before dying, he tells Bridie to recover a notebook hidden on the Isle of Man containing crucial information. Miller is wounded but escapes. When Bridie returns to her room, she finds Miller dying and receives from him the notebook’s location and a request to dispose of his body. This sequence sets Bridie on a dangerous path as she navigates loyalty, love, and the shadowy world of wartime espionage.
With the German contact now at risk, Bridie heads to Liverpool and then toward Ireland, as British intelligence pieces together the clues, always one step behind. Travel to Ireland is suspended, so she presses on to the Isle of Man, where she deciphers the notebook and learns the location of the impending D-Day landings. She decides to burn the notebook to keep its contents out of enemy hands. David, though, narrowly saves Bridie from arrest as a Miller confidant and confesses his love; she tells him what she has done.
Bridie tries to turn herself in, but the German agents kidnap her; David tracks them and is abducted as well. The two are taken to Ireland aboard a fishing smack and then, during a border crossing, a tense melee erupts as smugglers posing as mourners help them slip into Northern Ireland. A mistaken belief that they remain in neutral Ireland leads David to urge Bridie to flee across the border, but she chooses to face the consequences. A BBC broadcast then announces that D-Day has begun, which changes everything. David helps her escape again, then discovers the spies in a room upstairs; a fight erupts, the police arrive, and all are arrested.
After the war, Bridie and David marry, but their honeymoon at the Cromwell Arms is marked by a lingering unease about the past and the danger they survived; the very place that bears Cromwell’s name becomes a symbol of memory that she cannot easily escape.
