Dr. Charles Pommer, a scientist attached to the United States space exploration program, is bent on proving that life exists in outer space. To this end, Pommer sends a satellite, Space Master X-7, into space and then brings it back to Earth intact. After months of orbiting, the satellite returns, crashing on California's desert floor, where a canister containing samples of microscopic life from outer space is extracted and taken to Pommer for examination. Pommer, carrying the canister, is escorted to his home laboratory by Joe Radigan, a former research assistant of the doctor's who is now serving as a private in the army. When they reach the doctor's suburban house, the doctor brusquely dismisses Radigan. Inside, Pommer is surprised to find Lora Greeling, his embittered ex-lover. Lora, who has ceded custody of their illegitimate son to Pommer, tells the scientist that she has married a prosperous Honolulu businessman and is now in a position to rear the boy. When Pommer, impatient to begin his work, insists on delaying the discussion about their son's welfare, Lora begs him to hurry because she must return to Honolulu before her husband, who is ignorant of her past alliance, comes home from a business trip. Ignoring Lora's entreaty, the coldly selfish Pommer opens the canister and finds a fungus he names "blood rust." As Lora waits impatiently, Pommer dictates his findings into a tape recorder, observing that the fungus is rapidly multiplying and may be carried by anyone coming into contact with it. After a heated argument, Pommer, who has sent the boy to boarding school, agrees to relinquish custody to Lora, who then calls a cab to drive her to the airport. Later that night, John Hand, the head of project security, receives a desperate call from the incoherent Pommer, cautioning that his lab must be destroyed and burnt to the ground. Speeding to Pommer's house, Hand and Radigan discover the fungus oozing throughout the lab and over Pommer's dead body. After retrieving the tapes, they set the house on fire and then proceed to the decontamination chamber. Aware that the fungus can be spread through human contact, Hand listens to the doctor's tapes and when he hears Lora's voice in the background, realizes that someone else has been exposed to the fungus. Hand and Radigan's search for the missing woman leads them to the taxi agency, where the driver recalls that his passenger took a train bound for Los Angeles. As Hand and Radigan fly to Los Angeles, Lora's train arrives at Union Station. Spotting a newspaper headline detailing Pommer's death and the search for the missing woman, Lora assumes that she is suspected of murder and flees. Upon arriving in Los Angeles, Hand and Radigan are met by Chief Hendry. When the fungus is found in the luggage compartment of Lora's train, the compartment is decontaminated and a bag matching the cab driver's description of Lora's suitcase is extracted. Inside the suitcase, they find an airline ticket from Honolulu. The cab driver is then brought to Los Angeles to work with a police artist on a composite drawing of the mystery woman. When Lora sees the drawing on a television bulletin, she takes refuge in a small hotel, registers under the alias Alice Thompson, changes her hair color and sends for a new wardrobe. Soon after, Lora is notified by the airline that a seat is available on the next flight to Hawaii. Hand and Radigan, meanwhile, have been alerted by the hotel clerk that a woman resembling the drawing is a guest at the hotel. By the time they reach the hotel, however, Lora is gone, but from a can of hair dye and empty clothes boxes that she left behind, they deduce that the woman is now a brunette wearing a tweed coat. Hurrying to the airport, Radigan learns that a woman matching Lora's description has boarded the Honolulu-bound plane. Racing aboard at the last moment, Radigan discovers that there are three passengers who approximate the description. Radigan begins to question each woman, and when one balks, he tells her about the fungus. After the woman relates the story to Lora, Lora confesses that she is the one they are looking for and Radigan assures Lora that her private life will remain secret. As Radigan orders the plane to turn around, the fungus begins to seep from the baggage compartment and plans are made for an emergency landing. The fungus then encases the outside of the craft, causing the landing gear to freeze, and the plane is forced to make a belly landing. Once on the ground, the passengers are sequestered and taken to decontamination.