The film begins with the opening words of Hemingway's story: "Kilimanjaro is a snow-covered mountain 19,710 feet high, and is said to be the highest mountain in Africa. Its western summit is called the Masai 'Ngje Ngi,' the House of God. Close to the western summit there is the dried and frozen carcass of a leopard. No one has explained what the leopard was seeking at that altitude."

The story centres on the memories of disillusioned writer, Harry Street, who is on safari in Africa. He has a severely infected wound from a thorn prick, and lies outside his tent awaiting a slow death, though in the film it is pointed out he may have acquired the infection from leaping into a muddy river to rescue one of the safari's porters from a hippo after he falls in the river. His female companion, Helen, nurses Harry and hunts game for the larder.

The loss of mobility brings self-reflection. In an often delirious state he remembers his past relationship with Cynthia Green, whom he met in Paris as members of the "Lost Generation." Upon the sale of Harry's first novel, rather than rent a nicer home, Harry wishes to go on safari to Africa. There he has his happiest moments, including bagging a rhino. Cynthia is pregnant, but worries about sharing this news with Harry, who is passionate about his travels and work as a journalist and author. Harry only learns about the pregnancy after her miscarriage. Suffering depression and sinking into alcoholism, she eventually leaves Harry for a flamenco dancer when she believes Harry is off for a job as a war correspondent.

Harry later becomes engaged to the wealthy and socially connected Countess Elizabeth, whom he meets on the Cote d'Azur; however, he still remains loyal to the memory of Cynthia. On the eve of their wedding, a jealous Elizabeth confronts Harry with a letter to Harry sent from Cynthia, who is now in Madrid. Elizabeth destroys the letter in front of Harry who stalks off to go to Spain. Unable to find Cynthia at the Madrid address on the envelope, he enlists to fight in the Spanish Civil War. During a battle he meets Cynthia, who is now an ambulance driver. Cynthia is mortally wounded, and Harry is shot and wounded when he deserts the battle to try to bring the dying Cynthia to a doctor.

Only The Valiant B&W

Following the American Civil War, peace is maintained in the New Mexico Territory by Fort Invincible, a fortification set up outside a mountain pass that blocks marauding bands of Apache. The Apache are able to eventually take the fort by cutting off its water supply, then assaulting the fort when its garrison is at its weakest and killing all the defenders.

Captain Richard Lance arrives with a patrol soon after the battle and captures Tucsos, the charismatic leader of the Apache. Lance's scout advises the captain to kill Tucsos, but Lance will not shoot a prisoner.

Back at the headquarters of the 5th Cavalry, the invalid commanding officer orders Lance to assign an officer to command an escort to take Tucsos to a larger post. Lance decides to lead the patrol himself, but at the last minute, the colonel says he needs Lance to stay at the fort in case of an Apache attack, and orders him to assign another (but more popular) officer, Lieutenant Holloway, to lead the small group of men escorting Tucsos. The Apache free Tucsos and Lieutenant Holloway ends up dead. The men at the fort blame Captain Lance, unaware of the colonel's order. They believe that his decision to assign Lieutenant Holloway to the dangerous mission was for a personal reason (both officers were vying for the affection of Cathy Eversham, an officer's daughter). Cathy Eversham believes it too, and bitterly breaks up with him.

Lance's standing with the soldiers at the fort only gets worse when he assembles a group of misfit cavalrymen to hold off the rampaging Indians at the ruins of Fort Invincible, which is considered a suicide mission.