Arriving at New York City's Pennsylvania Station after a trip to Cuba, Sheila Bennet (Evelyn Keyes), who is smuggling $50,000 worth of diamonds into the country, realizes she's being followed by the authorities. She mails the diamonds to her husband, Matt Krane (Charles Korvin), instead of carrying them around, and then tries to shake the Treasury agent following her.
Feeling sick, Sheila nearly faints on the street, so a cop takes her to a local clinic. While there, she encounters a little girl and inadvertently infects her. Sheila is misdiagnosed as having a common cold, and she leaves and returns home. After the girl is admitted to the hospital, she is found to have smallpox.
Meanwhile, Matt has been cheating on Sheila with her sister, Francie (Lola Albright), and then attempts to take off without either of them when the diamonds finally arrive through the mail. Unfortunately for him, the fence cannot buy the diamonds because they are too hot. Matt will have to wait for ten days for the cash, so he cannot leave New York. Sheila confronts Francie, who kills herself afterward due to Matt's betrayal of them both. This gives Sheila more reason to get revenge on him.
Finding a growing number of smallpox victims, city officials decide to vaccinate everyone in New York to prevent an epidemic, but quickly run out of serum. This causes a panic in the city. Tracking the victims, agents realize that the disease carrier and the diamond smuggler are one and the same. However, an increasingly sick Sheila continues to elude capture. Still unaware that she has smallpox, she returns to the doctor at the clinic to get more medicine. The doctor explains her illness and tries to talk her into turning herself in, but she shoots him in the shoulder and escapes.
Sheila eventually catches up with Matt, who tries to escape from the police, but falls from a building ledge to his death. Sheila nearly attempts to drop herself from the ledge, until the doctor tells her the little girl she met had died. Remorseful, Sheila turns herself in and, before succumbing to the disease, provides authorities with a badly needed list of those she contacted.Evelyn Keyes as Sheila Bennet
Charles Korvin as Matt Krane
William Bishop as Dr. Ben Wood
Dorothy Malone as Alice Lorie
Lola Albright as Francie Bennet
Carl Benton Reid as Health Commissioner Ellis
Ludwig Donath as Dr. Cooper
Art Smith as Arnold Moss
Whit Bissell as Sid Bennet
Roy Roberts as the Mayor
Connie Gilchrist as Belle
Jim Backus as Willie Dennis
Richard Egan as Treasury Agent Owney
Harry Shannon as Police Officer Houlihan
Beverly Washburn as Walda Kowalski - six-year-old
The New York Times film critic, Bosley Crowther, gave the film a mixed review, writing, "But, unfortunately, the script of Harry Essex, based on a factual magazine piece, has a bad tendency to ramble and to confuse two separate hunts. And the performances of the principal characters, while adequate, have little punch. Evelyn Keyes, as the fugitive smallpox carrier, manifests great discomfort and distress, but she is no more than a melodramatic cipher in a loosely organized "chase." William Bishop is blankly youthful as the physician and Charles Korvin is conventional as the lady's no-good husband who tries to give her the brush. Others are moderately effective in a potentially but not sufficiently intriguing film."
Film critic Dennis Schwartz gave the film a mixed review as well and wrote, "There's not much in the way of thrills or surprises in this minor film noir ... The action part of the melodramatic story was weakly told, while the noir characterizations of Sheila did capture the desperate feelings of the subject but it was not enough to overcome the overall inability of the story to have a heart to it. The city officials and Dr. Wood running around the city to stem the epidemic, seemed hard to fathom. The mechanical acting by everyone, except for Keyes, and the unconvincing action scenes made the film appear as the B film it was, despite the great noir camerawork of Joseph Biroc who caught how dark the city could be for someone on-the-run."