A strange prehistoric beast lurks in the depths of the Amazonian jungle. A group of scientists try to capture the animal and bring it back to civilization for study.
Having previously survived being riddled with bullets, the Gill-man is captured and sent to the Marineland of Florida in Florida, where he is studied by animal psychologist Professor Clete Ferguson (John Agar) and ichthyology student Helen Dobson (Lori Nelson). Helen writes a journal entry about the Gill-man that is dated 7/10/1954.
Following the Gill-man's escape from Ocean Harbor Oceanarium in Florida, a team of scientists led by the deranged and cold-hearted Dr. William Barton (Jeff Morrow) board the ''Vagabondia III'' to capture the creature in the Everglades. Barton is mentally unstable and apparently an abusive husband to his wife Marcia (Leigh Snowden), as he becomes very jealous and paranoid when Marcia is with other men. Their guide Jed Grant (Gregg Palmer) makes numerous passes on Marcia (which she constantly rebuffs), with Barton becoming paranoia about the two.