Kit [John Garfield] endured two years of brutal torture after being captured in the Spanish Civil War, withholding the vital information sought by his captors, particularly their leader, a never-seen Nazi with a limp. His lifelong friend, Louie Lepetino, arranged his escape. When Louie, a New York police lieutenant, dies under suspicious circumstances, Kit cuts short his convalescence in Arizona and returns to the city to investigate. At the end of the trip, he bumps into attractive fellow passenger Toni Donne [Maureen O'Hara].
When Kit goes to the police to find out what they know, Inspector Tobin [John Miljan] tells him Louie's death was an accident, but Kit knows better. After arranging to stay in the apartment of another friend, Ab Parker [Bruce Edwards], he begins to make the acquaintance of the various guests at the party at which Louie made his fatal plunge. Among them are Dr. Christian Skaas [Walter Slezak], a wheelchair-using Norwegian historian; his nephew, Otto [Hugh Beaumont]; Kit's old flame, Barby Taviton [Patricia Morison], who hosted the ill-fated party; and Toni Donne. Also present were singer Whitney Parker [Martha O'Driscoll], and her piano-playing accompanist, Anton [John Banner].
Kit is shaken when Dr. Skaas discusses the supposed superiority of modern methods of torture over those of the past; it jibes too closely with what he endured. Kit does not know whom to trust, but he is attracted to Toni, and she to him. However, it turns out that Toni was the only witness to Louie's fall, which raises Kit's suspicions. Meanwhile, Kit repeatedly hears, or imagines he hears, the man with the limp, suggesting that he may not be fully recovered from his ordeal.
When Kit returns to the apartment, he is attacked in the darkness. He manages to gain the upper hand, and when he turns on the light, he discovers his assailant is Anton. Anton reveals that Kit was allowed to escape from Spain, and that he has been watched constantly ever since in the hope that he would betray himself. Kit's brigade killed a general who was very close to Adolf Hitler. Hitler vowed to get all those responsible and to hang the brigade's battle standard on his wall. Kit knows where the flag is hidden.
The next morning, Kit is awoken by a gunshot. He finds Ab dead in the next room, shot through the head. Again, Inspector Tobin insists it must have been suicide. However, Kit knows that, because of a childhood accident, Ab was terrified of guns.
In front of key witnesses, Kit gives Toni a medallion from the battle standard, mounted in a necklace—a declaration for all to see that he knows where the flag is. Toni begs him to give up what she considers to be just a "dirty rag", but Kit is determined to foil the "little man" in Berlin.
In the end, Kit insists that she choose. She agrees to help him get into Dr. Skaas's office during another party. Kit knows the man with the limp will show up. But first, he has to drink a toast with Skaas. Previously, when drinking with the doctor, Kit switched their goblets, joking about an old Borgia custom. He repeats the gesture, but Skaas is prepared: His own drink is drugged. While Kit searches the office, he hears the man with a limp. The door opens, and Skaas walks in, dragging one leg. Skaas taunts Kit with the truth: Otto killed Lepetino (who was investigating the Skaases for the Federal Government), he himself murdered Ab—and Kit has been drugged. However, Kit manages to shoot and kill the doctor and summon help before losing consciousness.
Toni explains that her 3-year-old daughter is being held hostage. Kit lets her go and arranges to meet her in Chicago.
Kit tells Tobin he plans to go to Lisbon to retrieve the flag and hand it over to “some of the boys.” “Brigades will be forming again.” Tobin asks if Kit is sure about Toni…
Cut to Kit and Tobin watching as Toni boards the Lisbon Clipper. On board, Kit stops her explanations, saying, “I wish there were a little girl.”
> I wish there were a little girl.
“Another sparrow fell,” Kit says, watching as police take her away.
