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 "Gill-man" that killed Carl's 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 Gill-man, who then gets briefly caught in one of the ship's draglines. Although it escapes, the Creature leaves a claw behind in the net, revealing its existence.