While searching for a desert guide in the West African village of Timbuktu, idealistic Englishman Paul Bonnard meets Prefect Dukas, a greedy official who helps him recover his watch from petty thief and prostitute Dita. Pitying Dita, Bonnard hands her his watch, prompting her to return his wallet with an innocent shrug. Later that night, Dukas finds Joe January, a rugged American guide who agrees to be his guide in exchange for Bonnard paying the hefty fines Dukas has levied on him as well as paying for his passage out of Timbuktu. The next morning, Joe eavesdrops as Bonnard preaches to Dita about sin and forgiveness. Dita is taken by Bonnard's sincerity, but Joe scoffs at the Englishman’s attempt to save the prostitute's soul. Dita begs to join him on the expedition, but Bonnard refuses, claiming the journey will be too dangerous. During the first few days of journeying on donkeys, Bonnard states that his father Jonathon, a missionary and archaeologist, was searching for a treasure on an expedition over ten years ago from which he never returned. Bonnard explains that he has come to find the treasure in hope of carrying out his father’s dream of establishing a refuge for the sick and needy. As they rest at a water hole one day, a mysterious caravan of native Tauregs approach, leaving the cloaked figure of Dita behind. When Joe threatens to leave them both if Dita does not return to Timbuktu, Bonnard offers him a third of the treasure to continue. After Bonnard explains that he has his father’s map directing them to the lost city of Timgad, where the treasure is hidden, Joe reluctantly agrees to the terms even with the "batty dame." Over a campfire that night, Dita recounts an abusive childhood that led to her prostitution and attempts to "cleanse" her skin with a knife. While Joe taunts Dita, Bonnard takes the knife and reminds her that tears cleanse better than knives. The next day, when Bonnard tries to help Dita as she is attacked by tarantulas, the two tumble down a hill in each other’s arms, prompting Joe to mock Bonnard for "pawing at” the woman he is trying to save. When they hear the Tauregs chanting nearby for a dying tribesman, Bonnard bravely offers the strangers his help and saves the man’s life, thus ensuring them safe passage. The next evening, Joe, softening toward Dita, offers her a drink then makes a pass. Hearing Dita’s protests, Bonnard shoots the bottle out of Joe's hand, causing a fight between the men, but they soon agree to be friends. The next day, when Bonnard gives Joe only approximate directions to follow, an enraged Joe tells them that being a couple degrees off could lead to their deaths from thirst and gives them eight hours to find the lost city. When the time is up, Joe orders them to return Enzeze, an eight-hour trip for which they have just enough water. As the men argue, Dita pours out the remaining water, telling them that she believes in Bonnard's dream. Joe is furious but has no choice but to continue on in search of water. Suddenly his donkey Janis leads them to a lush pool of water near the ancient ruins of a lost Roman city, where Bonnard finds three human skeletons lying among the ruins. Studying the skeletons, Joe notes that one of them, Jonathon, was killed by a gunshot wound to the head, while his guide was knifed in the back with his arms around a third skeleton, a woman. Joe then finds a love letter from Jonathon among the woman's possessions, which explains that he had asked her to join him in Timbuktu, told her of the treasure and promised to spend the wealth on a pampered life together in Paris. Dita tries to reassure Bonnard that he can continue with Jonathon's dream to build a refuge, but Bonnard, shattered by his father’s lie, starts drinking. After Joe buries the three bodies, a drunken Bonnard throws the bottle, startling a group of nearby bats. Later that night, after he finds the door his father described leads to an empty vault, Bonnard’s despair deepens. Meanwhile, Joe attempts to reconstruct Jonathon's last minutes: Realizing that his girl friend's alliance has shifted to the guide and fearing for his life, Jonathon moved the treasure somewhere else and gave his son a hint in the dog-eared page of his bible left beside his body, which alludes to “a day that man shall cast his gold and silver to the bats.” Deducing that the bats’ cavern must be the treasure's location, Bonnard lowers himself into the underground passageway where he finds gold, coins and jewels. The next day as they revel in the wealth, Bonnard takes Dita into his arms to kiss her, begging her and then violently demanding her to return his affections. After she struggles free, Bonnard showers her with jewels and persists, but Dita is so upset by the betrayal that she runs to Joe, who knocks Bonnard down. Convinced that, as with his father, the guide has taken the girl and will soon take his life, the hysterical Bonnard shoots at them and flees. As they wait through the night for Bonnard's return, Joe comforts Dita and chastises the “do-gooder.” The next morning, Joe and Dita discover Bonnard has fled with all the donkeys and provisions, leaving them to die. Filling sacks with water, they rush to track him down. After hours of walking in the scorching desert sun, they find Janis, but Dita faints, near death from thirst. Joe brings out his whiskey reserve and tells her that she is more beautiful than ever, prompting Dita to profess her love for him. Renewed by their love, Joe and Dita continue until they finally find Bonnard near death, clutching his treasure. As Joe and Dita dig for water, a deliriously paranoid Bonnard buries his treasure and stabs Joe in the back with his knife. Dita shoots and kills Bonnard, then digs until she hits water to give to Joe. Thinking that he is dying, Joe blames Jonathon's lies for destroying Bonnard, but thanks him for bringing him and Dita together. Seeing a Taureg caravan headed for Timbuktu, Dita embraces Joe, knowing that they will survive and start a new life together.