Plot
In 1942, during World War II, Wing Commander Max Vatan, a Royal Canadian Air Force pilot with intelligence duties, travels to Casablanca in Morocco to assassinate a German ambassador. To pose as a married couple, he is partnered with a French Resistance fighter, Marianne Beauséjour, who escaped from France after her resistance group was compromised and eliminated. Marianne secures Max an invitation to a party where they plan to conduct the assassination. Their mutual affection and a belief that they might not survive the day result in a sexual encounter on the day of the mission. After the successful assassination, they escape, and Max asks Marianne to come with him to London and be his wife. They marry, settle in Hampstead, and have a baby girl whom they name Anna.
In 1944, Max learns from the Special Operations Executive that Marianne may be a German spy who stole the identity of a French Resistance fighter, and that the German ambassador they assassinated was a dissident Hitler wanted dead. To test their suspicions, the SOE runs a "blue dye" tracking operation: Max is ordered to write down a piece of false intelligence at home, where Marianne can find it. If the information is picked up from intercepted German transmissions, Max must execute her, and if found to be an accomplice, he will be hanged for high treason. Defiant, Max visits Guy Sangster, a former colleague who knew Marianne; however, Sangster, blinded in the war, cannot confirm her identity. He reveals that resistance fighter Paul Delamare, who worked with Marianne in France, is still alive in Dieppe and could identify her. Max finds a young pilot, Adam Hunter, and gives him a photograph with a "classified" note—asking if the woman in the photo is Marianne Beauséjour—and instructs him to obtain a "yes" or "no" answer from Delamare. When Max and Marianne host a house party, his commanding officer, Frank Heslop, tells him that Hunter was killed while waiting on the ground for the answer from Delamare and berates him for his insubordination. Max wonders if what he was told about Marianne is a test of his loyalty as part of a promotion to V-Section.
The next evening, Max takes the place of a Lysander pilot and flies to France to meet Delamare. Max and the local resistance break into the town's jail, where a drunken Delamare, in custody, unconvincingly verifies the picture. Germans arrive, alerted by a French policeman, and are defeated by Max and the resistance. Before leaving, Delamare tells Max that Marianne was a talented pianist who had once played La Marseillaise in a café in defiance of occupying Germans. Back home, Max takes Marianne to a pub and demands she play the piano. She admits she is a spy and forwarded the "blue dye" message, which Max left in plain view. She insists her feelings for Max are genuine, and she was forced back into being a spy because German agents were threatening Anna. Max, unable to kill her, insists she flee. He kills Marianne's German handlers and drives to an airbase. Max starts a plane before Heslop and the military police arrive. He pleads his case; Marianne tells him she loves him, asks him to take care of Anna, then kills herself. Heslop orders the military police officers to report that Max executed Marianne as per his orders. The last shots of the film and a few photographs shown in this last sequence explain that, after the war, Max moved with his daughter Anna to a ranch in Medicine Hat, Alberta.
