Adventurers Martin and Osa Johnson lead a safari into the African Congo to explore the secrets of the "Dark Continent." After recruiting a team of guides, comprised of men from Nairobi, Kenya, the Johnsons begin what they call "the greatest jungle expedition of all time--in numbers, in duration and in the distances traveled." Their journey begins with a closeup view of animal life on the African plains, examining wildebeests, zebras, giraffes, flamingos, and hippopotamuses. Crocodiles are seen moving down a riverbank, and the roar of the mighty Murchison Falls is heard in the distance. The Johnsons mingle with a group of pygmies and follow native gorillas into their natural habitat. Sunrise on the Serengeti Plains finds wildebeests, gazelles and giraffes traveling in groups of thousands, and it is noted that all these animals are prey to the "King of Beasts," the lion. Wild flamingos remind Martin of chorus girls who, he states are also "long on legs but short on brains." Following the discovery of a baby giraffe, the Johnsons film a lion killing a zebra and a pack of wild dogs tormenting the lion as it attempts to eat its kill. Next, the rhinoceros is observed, as is the camel, which Martin refers to as "mean, cranky and dumb." As the Johnsons approach the Nakuru lake, flamingos are seen taking to the air. Later, the Johnsons photograph a Ndeze tribal village, a place that has never before been filmed. After listening to the sounds of the native Ndeze drums, the Johnsons travel to the Ripon Falls, head waters of the Victoria Nile, where Osa scares away a herd of elephants. In the Ituri Forest, in the Belgian Congo, Martin points out the dangers of sailing down a river rife with hippopotamuses poised to capsize their boat. While visiting a pygmy camp, Martin gives the pygmies cigarettes, which they try for the first time, and then watches with amusement as they choke on the smoke. After the Johnsons convince the pygmy chief to call in pygmies from throughout the region to come to their camp the following morning, the natives set out to spring elephant traps so that no one is injured. The next day, Osa entertains the visitors by playing jazz music for them and teaches them how dance to it. A few weeks later, and four hundred miles south of the pygmy camp, a gorilla is spotted and the Johnsons proceed with extreme caution. Two gorillas are captured and sent to the San Diego zoo. The film shows a colobus monkey and Teddy, a chimpanzee, being washed. Snowball, a young gorilla, and Baby, a young chimpanzee, are later seen sleeping in bed.