Geologist Dr. Carl Maia, on an expedition in the Upper Amazon basin, discovers a fossil embedded in some rock, it the claw of an unknown creature which demonstrates that whatever it was was powerful. With characteristics of the creature being amphibious, he consults with among others a former student, ichthyologist Dr. David Reed currently working for a marine biology institute in Brazil. Dr. Maia is able to convince not only David but others from the institute to join the expedition to unearth the reminder of the fossil, the team which includes David's boss Dr. Mark Williams, and Mark's research assistant Kay Lawrence, who also happens to be David's girlfriend. Mark agreeing to fund the expedition is on his ambition, he often taking the credit for any research of significance at the institute, that ambition which often places him at odds with David. When they work at the site of the initial find to try and find the remainder of the fossil without success, David begins to believe from the geological evidence that it may be further upstream of the tributary at what is known as the Black Lagoon. What they eventually discover is that such a creature still exists in the lagoon. The animosity that exists between David and Mark, which continues on this expedition, may prove to be a lethal combination for some or all of the team as they decide what to do about the potentially deadly creature.—Huggo