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In Milan, British secret agent Catherine Ullven receives microfilm showing the assassination of a prime minister. Because the microfilm must be conveyed to the United States, she visits a railroad car that holds two black panthers being transported to America by ex-patriot Steve Quain. After telling him that she is a reporter doing a story on the panthers, she secretly feeds them drugged meat. She then invites Steve to meet her later for a drink, and while he waits in a café, sews the microfilm into the collar of the male panther. Meanwhile, one of Catherine's allies is tortured until he reveals the plan for transporting the microfilm. Later, in Switzerland, some men detach the railroad car containing the panthers. Steve is able to jump clear, and as the car goes over the cliff, the panthers kill one of the men and escape. Steve is brought to a hotel where Captain Heimer tells him that sabotage caused the accident. Meanwhile, several other guests arrive at the hotel, all drawn there by the panthers. They include Chris Denson, a foreign correspondent from Great Britain; big game hunter Paul Kopel and his two hunting dogs; and Stephen Paradou, an artist. Later, as Steve is packing to return home, Catherine enters his room and explains why she is following the panthers. Steve is not interested in her story, but later, he saves her from a rough interrogation by a stranger, and agrees to stay in Switzerland to help her. After a soldier is killed by the panthers, searchers pursue them. Kopel's dogs pick up the trail, but as Kopel takes aim, Steve discharges his gun and scares the panther away before he can kill it. That night, everyone lies awake watching each other. About three in the morning, Kopel's prize dog breaks free and corners the panther, which kills her. While Kopel, Denson and a soldier search for her, Catherine and Steve set out on their own. They see one panther enter an abandoned cabin and manage to trap it inside. Before they can examine the collar, however, Kopel, Paradou and Denson arrive. Later, while some of the guests hunt for the remaining panther by moonlight, Paradou kills the caged panther and opens its collar. The dead animal is the female, however, and the collar is empty. When Catherine notices the slain panther, she hurries out to find Steve. Together, they flush out the male animal, and Steve kills it. He retrieves the microfilm, but just then someone shoots at him. While evading the bullets, he discovers Kopel's body. Soon Steve learns that Denson is trying to kill him, and Denson orders him to throw the collar into the open. Steve complies, and while Denson examines it, Steve runs at him. Denson shoots him, and is killed by Catherine. Catherine then takes Steve back to the hotel, where Dr. Stahl, who also runs the hotel, tends to his wounds. While Stahl and Catherine chat, Paradou sneaks upstairs to look for the film. Hearing a noise from Steve's room, Catherine and Stahl hurry upstairs. Paradou holds them at gunpoint while he awakens Steve and demands the film. He then threatens to kill Steve unless Catherine tells him where it is hidden. Because she has fallen in love with Steve, Catherine admits that the film is hidden in one of the rifle bullets. She carefully opens the bullets and empties the gunpowder on the table until she has made a small pile. She then lights the gunpowder with a cigarette, severely injuring Paradou. In Paris, after Catherine and Steve deliver the film, she reveals that it really was hidden in a bullet and only luck prevented Paradou from finding it. Steve then proposes to the willing Catherine.