Once the expedition arrives, David and Mark go diving to collect fossils from the lagoon floor. After they return, Kay goes swimming and is stalked underwater by the creature, who then gets briefly caught in one of the ship's drag lines. Although it escapes, the creature leaves behind a claw in the net, revealing its existence to the scientists.
Subsequent encounters with the Gill-man claim the lives of Lucas's crew members, before the creature is captured and locked in a cage aboard the Rita. It escapes during the night, attacking Edwin, who was guarding it. Edwin smashes the beast with a lantern, driving it off. Following this incident, David decides they should return to civilization, but as the Rita tries to leave, they find the lagoon's entrance blocked by fallen logs, courtesy of the escaped Gill-man. While the others attempt to remove the logs, Mark is mauled to death trying to capture the creature underwater, single handedly. It then abducts Kay and takes her to its cavern lair. David, Lucas, and Carl give chase, and Kay is rescued. The creature is riddled with bullets before retreating to the lagoon, where its body sinks into the watery depths.
The excavation of the area where Carl found the hand turns up nothing. Mark is ready to give up the search, but David suggests that perhaps thousands of years ago the part of the embankment containing the rest of the skeleton fell into the water and was washed downriver, broken up by the current. Lucas says that the tributary empties into a lagoon. Lucas calls it the "Black Lagoon", a paradise from which no one has ever returned. The scientists decide to risk it, unaware that the amphibious "Gillman" that killed Carl's assistants earlier has been watching them. Taking notice of the beautiful Kay, it follows the Rita all the way downriver to the Black Lagoon.
CREATURE FROM THE BLACK LAGOON
A geology expedition in the Amazon uncovers fossilized evidence from the Devonian period of a link between land and sea animals: a skeletal hand with webbed fingers. Expedition leader Dr. Carl Maia visits his friend and former student, Dr. David Reed, an ichthyologist. He works at an aquarium in California and has also been a guest at Maia's marine biology institute in Brazil for more than a month. Reed persuades his boss, the financially minded Dr. Mark Williams, to fund a return expedition to the Amazon to look for the remainder of the skeleton. The group goes aboard the tramp steamer Rita, which is captained by crusty old Lucas. The expedition consists of David, Carl, and Mark, as well as Reed's girlfriend and colleague, Kay Lawrence, and another scientist, Dr. Edwin Thompson. When they arrive at the camp, they discover that Maia's entire research team has been mysteriously killed while he was away. Lucas suggests it was likely done by a jaquar, but the others are unsure. In fact, the camp was attacked by a piscine amphibious humanoid, a living member of the same species from which the fossil originated. The creature, curious about the expedition, goes to the camp. When its sudden appearance frightens the members, they attack it, and in response the enraged creature kills them.