In London in 1940, Catrin Cole is hired by the Ministry of Information to help write scripts for propaganda films.

Her male colleagues need her to craft realistic dialogue for female characters, but they refuse to credit her and pay her less than a man's wage. Her husband Ellis – a war artist, Spanish Civil War veteran, and volunteer air raid warden – seems indifferent to her achievement in getting the job.

Catrin is sent to interview two twins named Lily and Rose Starling, who supposedly participated in the Dunkirk evacuation.

They tell her that they never reached Dunkirk because their boat broke down, although they did take on soldiers from an over-crowded tug returning the other way, causing newspaper reporters to believe they had come from the combat zone.

After Ellis tells her he cannot afford to keep them living in London, Catrin lies to the Ministry and claims that the Starlings did reach Dunkirk.

Her boss Roger Swain is impressed and greenlights the film, with Catrin seconded to the production company with extra pay.