The story unfolds in London during the year 1940, following Catrin Cole, portrayed by Gemma Arterton, who finds herself called for an interview at the Ministry of Information. There, she is tasked with crafting scripts for short informational films. Meanwhile, her husband, Ellis Cole, played by Jack Huston, is a war artist unable to showcase his work due to a leg injury sustained in the Spanish Civil War. Initially supportive of Catrin's newfound career, Ellis's attitude shifts when he realizes that she is now the primary breadwinner. Feeling threatened, he devises a plan to send her back to Wales under the guise of protecting her from the dangers of the London Blitz.
Their relationship reaches a boiling point following a confrontation with Ambrose Hilliard, interpreted by Bill Nighy, who requests alterations to Catrin's script. Rather than dwelling on the conflict, Catrin is redirected by her superiors to delve into a story about twin sisters Lily and Rose, who had set out to sail their father's boat for the Dunkirk evacuation. Upon meeting the sisters, Catrin discovers that they encountered mechanical issues shortly after departure, leading to their being towed back to port by a tugboat involved in the evacuation, thus mistakenly reported as having made it to Dunkirk.
Faced with the risk of losing her job and returning to Wales, Catrin resorts to deception, convincing her superiors to produce the film under the title The Nancy Starling. Alongside Tom Buckley played by Sam Claflin and Raymond Parfitt portrayed by Paul Ritter, they craft a narrative that continues to evolve. Even when their ruse unravels, the decision is made to proceed with the project as a fictional film rather than a documentary.
Hilliard receives an offer to play the role of a drunken uncle named "Uncle Frank," initially rejecting it out of pride. However, following the death of his agent, Sammy played by Eddie Marsan](/actor/eddie-marsan), during an air raid, Hilliard eventually accepts the role due to the urging of Sammy's sister, Sophie, enacted by Helen McCrory. As location shooting begins, the script demands rapid modifications, particularly after the Secretary of War, portrayed by Jeremy Irons, insists on the inclusion of an American character to enhance allure for U.S. audiences. The character is to be played by Carl Lundbeck, embodied by Jake Lacy, a charming Norwegian-American pilot filling the role despite facing challenges in the 'talkies' genre. Thankfully, Catrin's creative guidance and strategic editing help ensure the film's revival, including persuading Hilliard to tutor Lundbeck in acting.
During filming, an evolving camaraderie blooms between Catrin and Buckley, filled with both lighthearted banter and deeper affection. However, the delays lead her to miss the grand opening of Ellis's war art exhibition. In a moment of revelation, she confides in a crew member that she is actually Ellis's long-term lover, having assumed his name for appearance's sake. When Catrin returns home to London, she uncovers Ellis in an intimate encounter with a younger woman named Natalia Ryumina, leading her to end their relationship and return to Devon.
After discovering the truth about her relationship, Buckley proposes to Catrin, but she initially declines, resulting in a heated argument. Yet, later in London during interior filming, she has a change of heart, and the two share a passionate kiss, just before a mishap occurs. While heading to consult on Hilliard's character, Buckley tragically is killed by a falling lighting rig while looking back at Catrin.
With the ultimate Americanized ending added, Catrin suggests that one of the twins, Rose, take responsibility for fixing the boat's propeller, fulfilling an idea she had championed from the onset. Overcome with grief and shock, Catrin withdraws from film but is motivated to return by Hilliard, who seeks her assistance for engaging roles now that he has come to terms with moving away from matinee idol parts. Attending a screening of the film, Catrin sees that a clip of her and Buckley has been incorporated into the closing scene. After conversing with moved audience members, she resolves to sit down and pen her next film.
