Fact1: Devonian period provides direct link between land and sea animals, Fact2: Reed persuades boss to fund return expedition to Amazon to look for remainder of skeleton, Fact3: expedition consists of David Mark as well as Reed 's girlfriend and colleague Kay Lawrence and scientist, Fact4: perhaps thousands of years ago part of embankment containing rest of skeleton fell into water, Fact5: Subsequent encounters with Gill-man claim before creature is captured in cage aboard Rita
Output: A geology expedition in the Amazon uncovers fossilized evidence a skeletal hand with webbed fingers from the Devonian period that provides a direct link between land and sea animals. Expedition leader Dr. Carl Maia Antonio Moreno orders his two assistants to stay in camp while he visits the marine biology institute. Maia reunites with his friend and former student, ichthyologist Dr. David Reed Richard Carlson. Dr. Reed works at an aquarium in California, but more recently he has been a guest at Maias institute in Brazil to study lungfish. Reed persuades his boss, the financially minded Dr. Mark Williams Richard Denning, to fund a return expedition to the Amazon to look for the remainder of the skeleton. Soon after Dr. Maia leaves camp, a piscine amphibious humanoid, a living member of the same species from which the fossil originated, becomes curious about the expeditions camp. When its sudden appearance frightens the assistants, they panic and attack, and in response the enraged creature kills them both. The group goes aboard the tramp steamer Rita, captained by crusty Lucas Nestor Paiva. The expedition consists of David, Carl, Mark, as well as Reeds girlfriend and colleague, Kay Lawrence Julie Adams, and another scientist, Dr. Edwin Thompson Whit Bissell. When they arrive at the camp, they discover Maias assistants have been killed while he was away. Lucas suggests it was likely done by a jaguar, but the others are unsure. A further excavation of the area where Carl found the fossil 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. Carl says 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 Carls assistants has been watching them. Taking notice of the beautiful Kay, the creature follows the Rita all the way downriver to the Black Lagoon. Once the expedition arrives, David and Mark go diving to collect rock samples from the lagoon floor. After they return, Kay goes swimming and is stalked underwater by the Gillman, who then gets briefly caught in one of the ships drag lines. Although it escapes, the creature leaves a claw behind in the net, revealing its existence. Subsequent encounters with the Gillman claim the lives of Lucass crew members, before the creature is captured and locked in a cage aboard the Rita. It escapes during the night, attacking Thompson, who was guarding it. Kay smashes the creature with a lantern, driving it off, but Thompson is severely injured. Following this incident, David decides they should return to civilization. Mark, obsessed with capturing or killing the creature, objects. As the Rita tries to leave, they find the lagoons entrance blocked by fallen logs, courtesy of the Gillman. While the others attempt to remove the logs, Mark is mauled to death while trying to capture the creature singlehandedly, underwater. 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.