Dr. David Reed was an icthyologist who worked for the Instituto de Biologia Maritima in South America. He was romantically involved with Kay Lawrence, although the two never had plans to marry. When Dr. Carl Maia came to the institute with evidence of the fossilized remains of a creature that was half-man, half-fish, David agreed to help him persuade Dr. Mark Williams to fund an expedition.
The party, which consisted of Kay, Mark, Carl, Dr. Edwin Thompson, Lucas and his crew, and David himself, were unable to find further fossil evidence in the tributary that Carl was excavating, although they did find Carl's men Luis and Tomas, left to watch over the campsite, brutally murdered. Lucas told them that the tributary emptied into the Black Lagoon, a "paradise," so David and Mark unanimously agreed to move the expedition there.
They soon wished they hadn't, for the fossil discovered by Carl had a living descendant in the humanoid fish-creature that would become known as the Gill Man. David and Mark clashed over whether they should leave or try to capture the Gill Man, while said creature killed Lucas' crewmen Chico and Zee and made several attempts to abduct Kay. Eventually David won by default when the Gill Man also killed Mark, but the entrance to the Black Lagoon was blocked by a fallen tree, courtesy of the Gill Man.
David himself removed the blockage, but the Gill Man kidnapped Kay while everyone was preoccupied assisting David with the winch. David followed the Gill Man back to his lair and managed to get Kay away from him, only to be attacked by the enraged monster. Had it not been for the timely arrival of Carl and Lucas, both armed with rifles, it's doubtful that David would've come out of his scrape with the Gill Man in one piece.