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In 1943, General Charles Larraby of British intelligence receives a message from agent "Rembrandt" in Holland, saying that his code has been broken.  Meanwhile, in occupied Holland, Dutch intelligence officer Col.  Pieter Deventer, the spy known as Rembrandt, is arrested by Nazi officers and taken to see Colonel  Helmuth Dietrich, who offers to free Pieter if he will become a double agent.  German headquarters are then raided by a local Resistance group under the command of a man known as "The Scarf," and Pieter is rescued.

The Scarf and his men take Pieter to their hideout in a sawmill, and The Scarf says he carried out the assignment as a favor to the British.  Just then, a radio bulletin reports Pieter's rescue and announces the names of three prominent Dutch citizens who will be killed if Pieter is not returned to German custody by the following morning.  At the appointed time, Pieter is still at large, and the executions are carried out.

In England, six months later, shortly before the invasion at Normandy, Pieter visits his commander, General Ten Eyck, who informs him that Larraby wishes to send an agent into Holland to act as a liaison between British intelligence and The Scarf.  Ten Eyck says the plan is to have Carla Van Oven, a Dutch woman living in England, impersonate music teacher Fran Saylors, who has been collaborating with the Nazis for some time.  Peter voices concern over Carla's character, pointing out that she had dabbled in the black market and associated with Nazis before her husband was killed.  Armed with her file, Pieter goes to Carla's apartment and confronts her about her questionable past.  Carla insists that she loved her husband and wants to work for her country to avenge his death.

Carla goes through an intensive training program for spies, but is injured in a parachute jump.  Fearing for her safety, Pieter urges Carla, whom he has come to respect, to give up espionage, but she replies that her work has become her reason for living.  Pieter and Carla kiss, and soon are involved in a serious romance.

The time comes for Carla to carry out her mission, and, disguised as Fran, she parachutes into the Dutch town of Arnhem.  Carla is met by The Scarf, who tells her that Fran has been killed for collaborating.  A lone wolf who fights Nazis for the excitement, The Scarf balks at the idea of working for the British, but admits his attraction to Carla.  The Scarf takes Carla to her quarters and gives her a passionate kiss goodbye as the landlady, Frau Gilder, looks on.  Behaving in character, Carla then has her landlady's grandson Jan remove the picture of Queen Wilhelmina from her room.

Carla is soon visited by Captain Von Stanger, who requests her presence at a Nazi dinner party the following week.  Resistance forces attack the party, and Jan is about to kill Carla when The Scarf intervenes.  The following morning, Carla attempts to give The Scarf an assignment from London, but he declines to cooperate.  Instead The Scarf takes Carla to his family home to meet his mother, to whom he is extremely attached.  The Scarf is devastated to see that his mother has had her head shaved—the punishment for collaborators—and his brother Chris says the local villagers suspected her of being involved with a German soldier.  Enraged, The Scarf tells Carla to inform the British that he will fight under their orders.

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One night, Jan is badly injured during a raid, and before dying, tells Carla and Frau Gilder that they were ambushed.  The Allies liberate Holland, and Pieter is sent to Arnhem to assist The Scarf, whose organization has been suffering remarkably high losses since joining forces with the British.  Larraby suggests that Carla is betraying the Resistance group to the Nazis and asks Pieter to handle the situation.  Meanwhile, Carla tells The Scarf she believes he is being betrayed, and begs him in vain not to go on that night's raid.

Late that night, Pieter surprises Carla in The Scarf's room and tells her the latest raid has been a disaster.  Unaware of Pieter's suspicions, Carla says that The Scarf always conducts his raids by the light of a full moon, and that his men are attacked by snipers rather than artillery.  She concludes that The Scarf has engineered the ambushes in such a way that he will not be killed.  Pieter scoffs at her theory, and when Carla realizes that he thinks she is behind the ambushes, she is crushed.  She convinces him she is not capable of such betrayal, and assures him that The Scarf will once again emerge unscathed.  At that moment, the surviving Resistance fighters return and report that The Scarf has been wounded and captured.  Pieter bitterly places Carla under arrest and leaves her in the custody of The Scarf's men.  However, when the Germans later ambush thousands of British paratroopers and seal off Arnhem--with The Scarf safely in a hospital behind enemy lines--Pieter begins to wonder if Carla's story is true.

Dietrich is captured, and Pieter visits him in prison and repeats the offer the German made him earlier: freedom in exchange for being a double agent.  Dietrich accepts, and they go behind German lines and remove The Scarf from the hospital.  Pieter then kills Dietrich and escapes with The Scarf in a plane.  Back in Holland, Pieter informs The Scarf of Carla's allegations, and The Scarf vows to kill her.  After The Scarf boasts that he can easily get into Arnhem and maps out the route he will use, Pieter proposes that the British use the route to smuggle out their paratroopers.

The mission is a success, and Pieter accuses The Scarf of betraying his men to avenge the wrong committed against his beloved mother.  Pieter demands to see where The Scarf was shot by the Nazis, and The Scarf rips off his bandages to reveal a healthy body.  The Scarf then attempts to flee, but is shot to death by Pieter and the British agents.  The rescued paratroopers return, and Pieter is thrilled to see Carla among them.