In the midst of World War II, in the year 1942, Max Vatan (Brad Pitt), an intelligence officer for the Royal Canadian Air Force, is tasked with a perilous mission in Casablanca, French Morocco: to assassinate the German ambassador. He finds an unexpected ally in Marianne Beausejour (Marion Cotillard), a resourceful French Resistance fighter who has spent two years in Morocco after fleeing France when her resistance group was betrayed.
Marianne, possessing deep connections among the German military and local French circles, is instrumental in helping Max infiltrate an upcoming party hosted by the ambassador. The two create a facade of being a married couple, which allows them to navigate through the perils of their treacherous mission. Although they understand the risks that emotions pose in their line of work, they find themselves growing closer as they adapt to their roles. Marianne educates Max on local customs, such as the peculiar tradition of husbands sleeping on the roof after intimacy with their wives, emphasizing that these details are crucial to their survival. She takes great care to ensure that Max's French pronunciation is impeccable as he poses as a local.
As they strike a balance between maintaining appearances and fulfilling their mission, Max is nearly exposed when a German officer recognizes him. In a tense moment, he must confront and eliminate this threat to safeguard his identity. Enchanted by Marianne's spirited nature in stark contrast to his more reserved demeanor, Max falls deeply in love with her.
With Marianne’s connections, she secures Max an invitation to the critical party where their assassination plot is set to unfold. As the clock ticks toward their grim appointment, they share intimate moments, culminating in a passionate encounter inside a car amid a raging sandstorm, fully aware of the potential consequences they may face. Their mission proceeds as planned, and they manage to escape after a nail-biting close call with danger.
After the successful mission, Max asks Marianne to join him in London as his wife. Their union is approved by Max's superior, Frank, although Frank offers a foreboding warning: marriages born of wartime are notoriously fragile. They marry, settle in Hampstead, and welcome a daughter, Anna, under the harrowing backdrop of a German air raid during childbirth.
Fast forward to 1994, Max enjoys a serene life with Marianne and Anna, but this peace is shattered when the Special Operations Executive (SOE) informs Max of their suspicions regarding Marianne’s true identity. She is believed to have taken the identity of the original Marianne, who perished in France. The ambassadorship, which Max thought was a legitimate target, turns out to be a political dissenter sought by Hitler. The SOE intercepts suspicious transmissions linking back to Max’s home, originating from a woman—prompting a harrowing examination of loyalty and truth.
In a twisted test of allegiance, Max is ordered to plant a false piece of intelligence in his home, the implications of which weigh heavily on him; failure to comply could lead to his execution for treason. In defiance of orders, Max seeks out Guy Sangster (Matthew Goode), an old comrade who, despite being unable to confirm Marianne's identity due to blindness, provides crucial leads. Max also enlists a young pilot, Adam Hunter, to secretly investigate Marianne's past with Paul Delamare (Thierry Frémont) in France.
Tragedy strikes when Adam is killed before he can deliver word back, and the fabric of trust unravels as Max grapples with the fear of betraying Marianne. To seek answers, he embarks on a dangerously bold mission to confront Delamare in France, ultimately affirming unsettling truths about Marianne’s past that culminate in perilous confrontations with local authorities.
Back in England, Max insists on testing Marianne’s abilities at the piano, only to unravel another layer of deception; she admits to being a spy while professing her genuine feelings for him. With danger closing in from all sides, they determine to escape together, but their plans fall apart when the SOE intervenes. In a tragic twist, Marianne takes her own life after imploring Max to care for their daughter, leaving him with a haunting burden of survival.
After the war's conclusion, Max realizes his childhood dream by moving to a ranch in Medicine Hat, Alberta, Canada, where he raises Anna and carries the weight of past decisions, eternally linked to the memory of Marianne, whose true identity remains encapsulated in a letter she had penned for their daughter, anticipating the day the truth would surface.
