Searching for rare blue diamonds that could enable a revolutionary communications laser, TraviCom employees Charles Travis and Jeffrey Weems discover the ruins of a lost city near a remote
volcano in the
Congo jungle. Karen Ross, Charles's ex-fiancée and a former
CIA operative, and R.B. Travis, Charles's father and the CEO of TraviCom, lose contact with the team while tracking their progress at the company headquarters. A remote camera shows the camp destroyed and strewn with corpses, before a monstrous
ape destroys the camera. Travis asks Karen to lead another expedition to the site.
Primatologist Peter Elliott and his assistant Richard teach human communication to a
mountain gorilla named Amy, whose
sign language is translated into a digitized voice. Peter is concerned by Amy's drawings of jungles and the
Eye of Providence and wants to return her to Africa. Karen and Romanian philanthropist Herkermer Homolka join the expedition. The group flies to
Uganda and meets wilderness guide Monroe Kelly. Captain Wanta warns them not to trust Homolka and lets them proceed only upon receipt of a large bribe. On their journey via
Tanzania and then
Zaire, Monroe reveals that Homolka has led previous, disastrous
safaris in search of the "Lost City of Zinj". Their plane is shot down as they parachute into the jungle. A native tribe leads them to Bob Driscoll, a wounded member of Charles's expedition who dies screaming upon sight of Amy. The group continues by boat, and they learn that Homolka believes Amy can lead them to the mine. They find the ruined camp near the City of Zinj. Richard and some porters are killed by a gray gorilla. The group keeps the gray gorillas at bay with automated sentry guns. At daybreak, they explore the city and surmise from
hieroglyphs that the inhabitants bred the gray gorillas to guard the mine but were overthrown by them. At the mine, Homolka begins collecting diamonds, only to be killed by the gray gorillas. Monroe, Karen, and Peter flee deeper into the mine, where they discover Jeffrey and Charles's bodies, with the latter still holding a giant blue diamond. Karen fits the diamond into a portable laser and uses it to kill several gray gorillas. The volcano erupts, and the four escape as lava floods the city, killing the gray gorillas. Karen reports to Travis, but when she learns that Travis was only interested in the diamond, she destroys the TraviCom satellite. They find a
hot-air balloon in one of Travis's wrecked cargo planes. Seeing Amy with fellow mountain gorillas, Peter bids her goodbye. The three take off in the balloon, and Karen has Peter throw the diamond into the jungle below. Amy watches the departing balloon, then joins the other mountain gorillas. ==Cast==