“The novel starts with an abrupt end to an expedition sent by Earth Resource Technology Services Inc. in the dense rain forests of Congo (Zaire) when the team is attacked and killed by unknown creatures and all contact with them is lost. The expedition, searching for deposits of valuable diamonds, discovered the legendary lost city of Zinj (in Arabic Zinj or Zanj refers to the southern part of the East African coast). A video image taken by a camera there, and transmitted by satellite to the base station in Houston, shows a peculiar race of grey-haired gorillas to be responsible for the murders.

Another expedition, led by Karen Ross, is launched to find out the truth and to find the city of Zinj, where there are believed to be deposits of a certain diamond, type IIb, which are naturally boron-doped and thus useful as semiconductors, though worthless as gemstones. This time the searchers bring along the famous White African mercenary Charles Munro, as well as a female gorilla named Amy, who has been trained to communicate with humans using sign language, and her trainer Peter Elliot. Time is of the greatest essence, as a rival consortium from corporations in Japan, Germany, and Holland are also searching for the diamonds, turning the entire expedition into a race to the city of Zinj. Unfortunately for Ross and her team, the American expedition encounters many delays along the way, including plane crashes, native civil wars, and jungle predators.

Eventually, Ross and her expedition reach the City of Zinj and discover the consortium's camp, like the original expedition's camp, in ruins and devoid of life. Ross and her team lose contact with the ERTS HQ due to a massive solar flare, then encounter the killer gorillas and are attacked. A brief battle ensues and several gorillas are killed. After studying the corpses and performing a rudimentary field autopsy, it is concluded the animals are not "true" gorillas by modern biological standards, but gorilla-human hybrids: their mass and height is closer to humans than gorillas, their skull is greatly malformed (the "ridge" that makes gorilla heads look "pointy" is nearly nonexistent) as well as their pigmentation is on the border of albinism: light gray fur and yellow eyes.

In addition, they exhibit different behavior: they are highly aggressive, ruthless and partially nocturnal. Unlike normal gorillas, the grey animals are also extremely social, a troop consisting of over a hundred, compared to a normal troop of a dozen animals. Elliot intends to name them Gorilla elliotensis after himself. Afterwards, Ross, Elliot, and Munro explore the ruins and discover that the killer gorillas were bred by the ancient inhabitants of Zinj to serve as guards to protect the diamond mines from intruders. After several more attacks, Elliot, with the help of Amy, finds a way to translate the language of the new gorillas (she refers to them as "bad gorillas") and piece together three messages ("go away", "don't come", "here bad"); they stop fighting the humans and become confused, leaving the camp.

Their victory is cut short by the eruption of the nearby volcano, accelerated by explosives placed by Ross for her geological surveys, that buries the city, the diamond fields and all proof of the "new" species under 800 meters of lava. Ross, Elliot, Munro, and the rest of the team's survivors are forced to run for their lives. The team then manages to find a hot air balloon in a crashed consortium cargo aircraft and uses it to escape. In an epilogue, it is revealed that Munro was able to retrieve a few hundred carats of the valuable diamonds and sold them to Intel for use in a revolutionary new computer processor, while Amy was reintroduced into the wild and was later observed teaching her offspring sign language.”