During World War II, former schoolteacher turned Dutch resistance fighter Vincent Van Der Lyn (Paul Henreid) causes so much trouble for the Nazis, they place a bounty on his head. As a result, he is ordered to travel to England by way of neutral Lisbon.
On Van Der Lyn's arrival, Police Captain Pereira (Joseph Calleia) notes that his passport has no exit stamp on it (indicating he sneaked across the border), but reassures the traveler that all that matters is that the Portuguese visa is in order. German agent Otto Lutzke (Kurt Katch) becomes suspicious and starts following the Dutchman.
At a restaurant, Van Der Lyn is pleasantly surprised when a beautiful stranger, Irene Von Mohr (Hedy Lamarr), sits down at his table. Irene had passed a card to a man in a nearby alley, only to see him shot in the back. She fled to the restaurant; when the police arrived to question everyone, she sat down to throw off suspicion. She describes herself merely as a frequent gambler at the Casino Estoril. She leaves, supposedly to make a telephone call, but never returns. The Dutchman goes to the casino and finds Irene. As she warns him to stay away from her, they are joined by Hugo Von Mohr (Victor Francen), who is a high ranking German diplomatic official, and Lutzke. The Germans soon identify Van Der Lyn as the saboteur nicknamed the "Flying Dutchman".
Van Der Lyn meets his contact, Ricardo Quintanilla (Sydney Greenstreet), who introduces him to other members of his resistance group: Pole Jan Bernazsky (Peter Lorre), Norwegian Anton Wynat (an uncredited Gregory Gaye), and Frenchman Paulo Leiris. Quintanilla asks him to brief Jennings (an uncredited Monte Blue), Van Der Lyn's replacement. In private, Quintanilla warns the newcomer that he suspects one of their group is a traitor.
The next day, when Irene gets into her automobile, Van Der Lyn invites himself along for the ride. At first annoyed, she gradually warms to him, and they spend the day together. He professes that he is in love with her. She tells him that she married Hugo after he rescued her from Dachau.
When he returns to his hotel room, he finds Jennings slumped over a desk. Jennings is able to give him a message before dying. Acting on a tip, the police arrest him for murder. A distraught Irene tells Captain Pereira that the Dutchman was with her all that day, but declines to testify in court. When she speaks with Van Der Lyn, he accuses her of framing him.
After he escapes, Irene finds him and offers to take him to Quintanilla, revealing that she too is a resistance fighter. His suspicions are allayed after she gives him a gun. When they reach Quintanilla and the others, they charge him with being a turncoat. He manages to convince them otherwise when he gives Quintanilla Jenning's dying message, which warns that his killers have taken the "eagle", a rare coin that was to have been used to identify him, and something that Van Der Lyn had not been told. Hugo is then revealed to be part of the underground group.
Quintanilla decides to set a trap, informing the others that Jennings' replacement is in the casino hotel, knowing that the Germans will have to eliminate him in order to successfully plant their own agent. Fifteen minutes before they are to meet the new man, Quintanilla reveals his room number, 865, to the others, gathered at a roulette table along with known Nazi agents. Pereira spots Van Der Lyn, but is persuaded to wait for the real murderer to reveal himself. With time running out, Hugo places bets on 8, 6, and 5. Quintanilla and the others escort him away, but he manages to escape. He is killed in a shootout with Van Der Lyn and Pereira. Van Der Lyn finds the eagle on his body.
Van Der Lyn decides to return to Occupied Europe in Jenning's place. Irene promises to wait for him.Hedy Lamarr as Irene Von Mohr
Paul Henreid as Vincent Van Der Lyn
Sydney Greenstreet as Ricardo Quintanilla
Peter Lorre as Jan Bernazsky
Victor Francen as Hugo Von Mohr
Joseph Calleia as Police Captain Pereira
Carol Thurston as Rosa, Miguel's daughter
Vladimir Sokoloff as Miguel, a Portuguese fisherman who helps Van Der Lyn
Eduardo Ciannelli as Police Colonel Almeida, Pereira's superior (as Edward Ciannelli)
Steven Geray as Dr. Schmitt
Kurt Katch as Otto Lutzke
Production on The Conspirators ran from late April to mid-May 1944. The film's working title was Give Me This Woman. In pre-production, a number of actors were considered for roles in The Conspirators, including: Joan Fontaine, Helmut Dantine, Humphrey Bogart and Ann Sheridan before Hedy Lamarr was borrowed from MGM for the film to star in the lead female role. It should be noted that Hal Wallis had asked Ayn Rand to help rewrite the love scenes in this film for director Jean Negulesco; apparently, only a few of Rand's lines were retained in the film, and she is uncredited as having contributed anything to the script. See a review by David Hayes.
Bosley Crowther, the critic of The New York Times, called The Conspirators, "a disappointing show. And, indeed, it would be quite as vexing if it came from a less able lot". Savaged by not only critics, The Conspirators was reviewed by Frederic Prokosch, the author of the novel on which the film was based, who wrote a brusque critique of it in The New Republic.