The film begins with TraviCom employee Charles Travis (Bruce Campbell), the ex-fiance of electronics expert Dr. Karen Ross (Linney), testing a diamond-powered communications laser in a remote part of the Congo situated on a dormant volcano. Charles' friend Jeffrey is excited over discovering the ruins of an ancient city, but when he enters a cave to retrieve his bag, he is killed and mutilated off-screen. Whatever killed him throws his eyeball at Charles and attacks from off-screen. Several minutes after the attack, Karen, who is waiting in the company's Houston headquarters for Charles to communicate via satellite, activates the video feed on his camera.Karen and her team are shocked to see a destroyed camp and several dead bodies. Before they can see much more, a shadowy animal rushes to the camera and knocks it over, ending the transmission. TraviCom CEO R.B. Travis (Baker), Charles' father, reveals that the purpose of exploring the Congo is to find a rare blue diamond expected to only exist at volcanic sites to expand his communication technologies. He orders Karen, who's former CIA, to finish what Charles started, namely, to find the diamonds. She makes him promise that he is sending her there for his son, and not simply for the diamonds.Meanwhile, Dr. Peter Elliott (Dylan Walsh), a young Primatologist at the University of California at Berkeley, and his assistant Richard (Heslov) have devised a way to teach human communication to primates using a gorilla named Amy (voiced by Shayna Fox). With a special backpack and glove, her sign language is translated to a digitized voice. Despite the success, however, Peter is concerned over the fact that Amy is having nightmares and psychological problems that have only been partly quelled by constantly painting the same picture of an eye surrounded by trees. Peter figures out that she is painting a jungle and decides to return her to Africa, but the university is reluctant to fund the expedition. Peter is then approached by a Romanian philanthropist, Herkermer Homolka (Curry), who offers to fund the expedition. Karen, hoping to find Charles, joins Peter and provides additional funding for his trip.Upon arriving in Africa, the group is met by their expert guide, Captain Munro Kelly (Hudson). However, they are soon captured by the local authorities and questioned by a military leader (Delroy Lindo) who grants them passage in exchange for a large bribe. As the group boards another plane, Munro reveals that Homolka has led previous safaris in search of the "Lost City of Zinj", with disastrous results. The group covertly crosses the sealed Zaire border but must parachute into the jungle after their plane is shot down by Zairean soldiers. They make camp in the jungle and Karen sets up equipment to contact TraviCom via satellite. Just as she begins the video feed to Travis in Houston, Peter and Amy, who were rough housing nearby, knock over the equipment abruptly. Travis, thinking this meant a second attack identical to the first, quickly sends in a third team. The next morning, members of a ghost tribe enter the camp, claiming to have seen a dead white man with the TraviCom logo on his clothes.The ghost tribe members lead the team to the man, a member of the original TraviCom expedition, Bob, who is not actually dead but in a catatonic state. As they attempt to revive Bob, he catches a glimpse of Amy and begins to scream uncontrollably, before coughing up blood and dying of shock. Perplexed, the group heads deeper into the jungle by boat, surviving a brutal hippopotamus attack at night. Along the route, they watch as a large cargo plane streaks across the night sky on fire, shot down by the Zaire military and doomed to crash in the jungle. Munro again presses Homolka about his obsession with the lost city, and Homolka reveals that as a young man, he found a book in Soviet Georgia that contained a detailed drawing of the City of Zinj, where King Solomon was believed to have had a vast diamond mine. The drawing featured a peculiar decoration that resembles an open eye, the same eye that Amy has been painting.Homolka comes to the conclusion that Amy has seen Zinj and can take the group there. The group finally discover the lost city of Zinj, while the dormant volcano shows signs of increasing activity. Peter, Karen, Munro, Homolka, and head porter Kahega (Akinnuoye-Agbaje) enter the ruins while Amy, Richard and the remaining porters wait outside. While the first group searches the city, they see that a certain hieroglyphic keeps repeating itself. Suddenly, a hysterical Richard runs into the city, screaming, blood on his face. He collapses dead and a gray gorilla jumps out of the shadows and throws the severed head of a porter at the group, the gorilla is quickly killed while another claws Munro, which is chased off by Kahega. Amy, then appears, clearly frightened. That night, they set up a secure camp, but the gray gorillas attack the perimeter after dark and are driven off by automated machine guns set up by Karen. Homolka translates the repeated hieroglyph: "We are watching you." In the morning, a few of the porters, Amy, and Homolka are missing.The group renter the ruins of Zinj, where the group finds Homolka and Solomon's diamond mine. Homolka picks up handfuls of large diamonds, but the group is quickly ambushed by the gray gorillas, who guard the mine and its precious minerals. Homolka is quickly killed by the gorillas and the others open fire with machine guns. In a small geode within the mine, Karen and Peter find Charles' dead body, a large blue diamond in his hand. Kahega and the other porters run out of ammunition and are killed, as gorillas pull Peter away from Karen and Munro and attack him.However, Amy arrives and fiercely protects the injured Peter from the gray gorillas, giving Karen enough time to load one of the diamonds into her own powerful laser, which she uses as a weapon on the gorillas. The volcano then erupts violently, collapsing the mine into molten lava and giving the three survivors time to escape as the killer gorillas are consumed by the boiling lava.Upon escaping the collapsing city and the erupting volcano, Karen contacts Travis and informs him that she found the diamond mine, but that Charles is dead. However, when she coldly realizes that Travis only values the diamonds, she programs the laser to target Travicom's multi-million-dollar satellite, destroying it in space. Having found the shot-down cargo plane, Karen has Munro find and set up a hot air balloon, something their own expedition had left behind at the airport. Meanwhile, Peter must say goodbye to Amy. She gives him a flower and sets off into the wild with a group of mountain gorillas. At sunset, Peter, Karen, and Munro set off in the hot air balloon. Karen has Peter throw away the only diamond she managed to save from the mine, and the balloon sails away from the jungle, as Amy looks on.