Code Name: Emerald

During World War II and a few months prior to the Normandy landings, Gus Lang, an Allied agent, is sent to occupied France in order to rescue an "overlord" captured by the Germans – one of the key people with intimate knowledge of when and where the D-Day invasion will actually occur. Several people will help him to succeed: a secret friend of the Allies; a vital German officer who is a highly placed mole; and the French resistance. The SS, however, will try to block their plans.

World War II, 1944. Nazi Germany knows the Allies will launch Operation Overlord – an all-out invasion of Nazi-occupied Europe via France. German Army Colonel Brausch, Hoffman in the Gestapo and Ritter in the S.S. have so far failed six times to capture someone who can tell them the actual D-Day date and landing sites in advance. Brausch offers the use of his spy in British Intelligence: US-raised Gus Lang, codenamed Emerald. But Lang/Emerald is really a double agent working for the Allies; he passes on fake details of a D-Day rehearsal ship carrying potential informants, and suggests that a heavily armed squadron of German fast-attack E-boats should intercept them.

Lang intends the E-boats to miss this invented ship; but his superior Colonel Peters decides to risk a real crew and their operational knowledge by holding a real D-Day rehearsal at Omaha Beach. Peters refuses to inform the military of his intelligence scheme, telling Lang he's protecting the identity of a disaffected high-level Nazi who is secretly working for the Allies. The German E-boats then attack the exercise: many Allied soldiers are killed in action and several are captured by the Germans, including Lieutenant Andy Wheeler, a U.S. Army signalman whose job has given him intimate knowledge of the D-Day landings.

Wheeler is interrogated at Gestapo HQ, an old castle in Paris, but stays silent; however, Ritter learns his name and rank by interrogating other Allied captives, and deduces from his signalman's job that the Nazis have captured an Overlord at last. Brausch, Hoffman and Ritter decide to bring the US-raised Lang/Emerald into their team to befriend Wheeler and get him to talk. Knowing that the British believe Lang to be their own agent, the Germans feel they have the upper hand. Before leaving England, Lang is instructed to report at once if Wheeler has talked. To keep Wheeler from betraying intelligence under torture, Lang "procures" Wheeler's army medical record for the Nazis, which British Intelligence have falsified to include a cardiac arrhythmia. Lang is also authorised to kill Wheeler to prevent him from talking.

Lang parachutes into France and meets his French Resistance contact Henri, who drives him to Paris in his fishmonger's van and instructs him how to secretly relay his reports to England. In Paris, Lang reports to Ritter, a fanatical anti-Semite, then meets with Hoffman and Brausch to discuss the interrogation. Wheeler is being disoriented via enforced sleep deprivation, irregular meals and frequently being moved between different cells. Now they will introduce Lang as a fellow American prisoner who must befriend Wheeler and persuade him to talk.

Lang evades his Gestapo tail in a Paris Metro train carriage to meet his go-between contact Claire Jouvet, who will pass on his reports. Knowing that Gestapo agents are following Lang, he and Claire pretend to be lovers, although it develops into real romance. They have sex for the first time at Claire's apartment, where they witness her elderly neighbour's retaliatory arrest by the Germans.

Lang starts befriending Wheeler by guessing successfully that he's a Texan. As Wheeler introduces himself by name, and they converse about baseball, Brausch, Hoffman and Ritter celebrate Lang's imminent success. But the naturally suspicious Ritter begins to investigate Claire's background. She has told Lang that she has a son by an absent father named Duchelle, whom Ritter arrests and interrogates, learning not only Claire's links to the Resistance but also – even more importantly for the Jew-hater – that she has a distant Jewish ancestor. Hoffman confides Ritter's actions to Brausch, saying that although Claire has technically committed a capital offence by not registering her Jewish blood, it is trivial.

Meanwhile, Lang gets Wheeler to reveal a decoy location for the D-Day landings at Calais, but tells Brausch, Hoffman and Ritter that he needs some truly private time with Wheeler to make sure this intelligence is trustworthy. Accordingly, Lang and Wheeler are let into the open castle grounds for an hour's exercise, despite Ritter's misgivings. There Lang reveals to Wheeler he's a double agent, mentioning what only the Allies know – that D-Day will target Normandy, not Calais. Lang then tells Wheeler to 'reveal' that the assault will be led by the decoy battalion FUSAG in early July – a month later than the actual D-Day plan. Lang determines to rescue Wheeler rather than executing him – especially as Wheeler's talk of his girlfriend reminds Lang of Claire.

Hoffman's and Ritter's suspicions of Lang are increasing, however. Hoffman poaches Lang's money-hungry handler Callaghan to the Gestapo by offering to pay him more than Brausch. Callaghan shows Hoffman Wheeler's unaltered civilian medical record. Ritter then intimidates Wheeler by saying the Germans know his 'heart condition' is faked. He shows him a photograph of Lang and Claire socialising with Nazis at a Parisian restaurant, and leaves him to contemplate the torture he can now expect.

But Lang suddenly discovers a new ally. Brausch calls Lang to a secret meeting in a church, where he reveals they have a mutual friend in Lang's superior officer, Sir Geoffrey Macklin – Brausch is the high-ranking Nazi defector Colonel Peters had been protecting. Now, he warns Lang of the Gestapo's suspicions of Claire, and that Callaghan was seen conversing with Hoffman. Brausch tells Lang that Hoffman will move against him, but that Hoffman's paranoia of betrayal means he will act alone.

Hoffman, Lang and Claire take a chauffeur-driven drive for a country picnic. On a private 'walk and talk', Hoffman draws a gun and tells Lang his cover is blown. A shot rings out – but Hoffman is the one who falls dead. Claire has shot him, and goes on to shoot the surprised chauffeur. Bidding an emotional farewell to Lang, Claire is driven by fellow resistance agent Henri to collect her son. They plan to rendezvous at 5pm at an airfield where Lang will bring Wheeler for extraction.

Lang dresses in his Gestapo senior officer's uniform and returns to HQ, where he hands Brausch a document bearing Hoffman's signature – cleverly forged by Claire – which removes Ritter's authority and charges Lang to bring Wheeler immediately to Calais where Hoffman is waiting. Brausch reads out the order to all officers present. Ritter arrives in protest, but Lang asserts his authority by flooring him with a punch. As the suspicious officers telephone for personal confirmation from Hoffman, Lang and Brausch slowly walk the exhausted Wheeler to the staff car.

Lang knows Brausch's life will be in danger once the Germans figure out the ruse, but Brausch refuses to be extracted, saying he can buy time for Lang and Wheeler. Their car leaves Gestapo HQ just as the alarm is sounded, leading the Germans on a chase into the countryside, where Henri picks them up in his fish van. The Germans are left to search the abandoned staff car.

At the airfield, the rescue pilot urges Claire to leave, as they can wait no longer for Lang and Wheeler. Resigned, Claire boards the plane with her young son… but the fish van arrives at the last moment. Once in the air and heading for England, Lang and Claire kiss passionately, making Wheeler smile. The film ends with brief real footage and commentary of the D-Day landings on 6 June 1944.

• Ed Harris as Augustus "Gus" Lang
• Max von Sydow as Jurgen Brausch
• Horst Buchholz as Walter Hoffman
• Cyrielle Clair as Claire Jouvet
• Helmut Berger as Ernst Ritter
• Eric Stoltz as Lieutenant Andy Wheeler
• Patrick Stewart as Col. Peters
• Graham Crowden as Sir Geoffrey Macklin
• George Mikell as Major Seltz
• Julie Jézéquel as Jasmine
• Katia Tchenko as Marie Claude
• Vincent Grass as Tracker