Paleontologist Joshua Green has no idea what he's getting into when his friend Lon Sanger shows him an ancient human skull coated in black ceramic, with gemstones for teeth and a huge emerald in one eye socket. When carbon dating suggests that it is older than mankind, Joshua prevails on Lon's smuggling connections to outfit an expedition to the remote Colombian jungle where it was found. Captured by cannibals, the survivors escape into a maze of tunnels and an underground city. There Joshua learns of the cycles of time that may now be coming to a premature end unless he can negotiate with the phantoms of the past.