In 1942, at the height of World War II, intelligence officer Max Vatan parachutes into French Morocco behind enemy lines. There, he teams up with French Resistance fighter Marianne Beauséjour on a deadly mission in Casablanca. Marianne, a skilled operative who has already infiltrated German high society, poses as Max's wife for the operation. Their objective is to assassinate the German ambassador during a high-profile party. To ensure the success of their mission, Max and Marianne spend time together creating a believable facade as a loving couple. As they meticulously plan the ambassador's assassination, their pretend intimacy blossoms into a real and deep love for each other.

Following the completion of their dangerous assignment, Max and Marianne relocate to London and marry, embarking on a new life together. They start a family amidst the ongoing war, but their seemingly blissful union is soon overshadowed by unexpected doubts. The pressures of wartime service weigh heavily on them, with the constant threat of espionage looming. Tensions rise as the British government's trust in Marianne comes into question, leading Max into a painful dilemma. He is faced with a harrowing task: to prove his wife's innocence against a charge of being a German spy, an accusation that carries with it severe and far-reaching consequences, forever altering the fabric of their lives and testing the strength of their bond.