A tributary of the upper Amazon. A small boat moves up river, it is late in the afternoon and the boat anchors as Captain Lucas (Nestor Paiva) announces they will push through to the Black Lagoon in the morning.
Two passengers, George Johnson (Robert Williams) and Joe Hayes (John Bromfield) debate the value of the expedition. Joe is determined to capture the creature and take it back to the Aquarium to exhibit it for the world to see.
Lucas warns that if it is dead they have wasted their money and if still alive, they probably will wish it was dead After dinner the discussion continues. Lucas cant understand the point of what they are doing. The creature should be left alone, not captured for the whim of science. Lucas explains he came to the lagoon the previous year with an expedition, and 5 people eventually were killed during that trip
They go on to discuss the origins of the creature. Johnson explains that under the right conditions a creature might avoid being affected by evolution, leap forging many important steps. Lucas argues the creature is too strong for evolution and if the two passengers think they can just capture the thing, they will be sadly mistaken
Next morning the boat enters the lagoon without incident. Unseen, the creature surfaces and studies their activities then swims off. Joe dons a diving suit and goes into the water to secure a net they have hung across the entrance of the lagoon
The creature sees Joe and stalks him, waiting for a chance to attack. Preoccupied with the net Joe doesnt see the creature till it is too late. The men on the boat hear Joes frantic cries for help and begin pulling him in with the life line. The creature tries to break the line before being driven off by gunfire from the boat
The next day Johnson sets a series of explosives around the lagoon, detonating them should stun the creature and force it to the surface. The plan works and the creature floats unconscious near the boat.
The men secure the creature and begin the task of transporting it back to Florida after notifying the press
At the institute in Florida Doctor Ferguson (John Agar) is studying predatory behavior in mammals when news comes through of the capture of the creature. He is encouraged to go to Ocean Harbor to study the creature
At Ocean Harbor the creature is unloaded from the boat and transferred to a holding tank so studies can begin. Concerns are raised over the fact the creature has remained in a coma since being stunned by the explosions. The publicity generated by the creature is enormous; hotels are book for 50 miles around by tourists wanting to see the creature
Overseeing the transfer is Helen Dobson (Lori Nelson) shes decided to use the creature as the basis of a thesis. She explains to reporters why Joe is walking the creature around the holding tank. The technique is the same used on sharks to push water through the creatures gills and revive it.
Two hours later the creature stirs. Tension mounts as Joe barely makes it out the tank before being attacked. The creature goes on a rampage and tries to jump out of the tank. Joe stops the creature but falls back into the tank. Two workers attempt to rescue him but are badly hurt before a net is thrown over the creature and it is bought back under control
Sometime later the creature is introduced to the main viewing tank so the public can see it. One leg is secured with a chain to stop the animals further attempts to escape
A few days later the aquarium has settle into a routine, visiting crowds are huge and the process of studying the creature begins. Ferguson and Helen meet and discuss their mutual fields of research and how it will affect the research being done. At the same time there is a clear emotional connection beginning to form between the couple.
As their conversation becomes more relaxed Ferguson asks Helen out to dinner. The discussion is interrupted by Joe Hays, also interested in Helen. She rebukes Joes advances and ultimately makes plans with Ferguson.
Time progresses and Ferguson and Dobson continue their experiments. They try training the creature using vocal commands similar to methods used on dolphins. Entering the tank, they are armed with a special shock probe; by zapping the creature at the right time during feeding they believe they can teach the animal to be less aggressive.
They then lower another portion of food that has been injected with drugs to make the creature go to sleep. Satisfied its pacified Ferguson attaches a device for measuring the creatures brain activity as well as drawing blood samples
The blood tests throw up some odd results, Ferguson begins to worry about exactly what they are dealing with. On every level tested the creature appears on an evolutionary scale closer to man than animals. Helen also expresses concern for the creature. She perceives it is lonely and worries what long term effects all the testing might have on it
The conditioning training continues and Ferguson is stunned by the creatures progress. What a chimp learns in three weeks the creature learns in a matter of hours. During one experiment the creature reacts by attacking and injuring Ferguson. Dobson pleads with the creature to stop, which it eventually does
However it has its blood lust up, and begins furiously attacking the chain holding it till a link breaks. Joe spots what the creature is up too and attempts to intervene. He is dragged into the tank and killed
Leaving the tank the creature sends park visitors scattering. Its behavior is unusual, although having the opportunity to kill a number of visitors it ignores them, preferring to concentrate on getting back to the nearby ocean
A week later reports are come from as far as Mexico and New York about possible sightings, however the authorities are convinced it is probably moving back to the habitat it knows best, the Amazon river.
That same night, Dobson readies for a shower in her hotel room. Unknown to her the creature has come ashore and is prowling around the building. Her dog attacks the creature as it enters her room. The noise disturbs Ferguson in the next room. He investigates only to Helen in the shower and her dog missing. Going out and checking the grounds he finds nothing. He assumes the dog has gone after a rabbit and should be back safe and well in the morning.
Unseen in the bushes are the remains of Dobsons dog, recently killed by the creature.
The next morning Dobson and Ferguson take a boat trip up river. There plan is to have lunch in Jacksonville as a farewell date. The creature knows she is onboard and begins following the craft till it breaks down. As the crew try to fix the fault Helen and Ferguson go for a swim
The creature follows Helen as she swims, but seems tentative about attacking other than briefly touching her leg. Before the aggression can become more marked, it is announced the boat is fixed and will soon be on its way.
The date then continues as planned. Taking a break from dancing, they go for a walk along the pier. Talk turns to the struggles the upcoming three month of separation will have on their relationship. As true scientist the subject turns to what love really is as they wander back to the dance arm in arm.
The creature climbs the pier and follows them back to the dance. Bursting into the room, it roars scattering the crowd then snatches Helen. Ferguson follows the creature to the waters edge then dives in frantically searching for her
A couple fishing in a boat nearby hear screaming coming from a channel marker in the river. Helen has broken free of the creature and is trying to escape by climbing onto the structure. As they watch she is dragged back under and despite the arrival of a police launch, is not seen again.
The police have issued an alert and begin clearing the public off the streets. Two young men driving find a body on a nearby beach. The stop to investigate and realizes it is Helen and she is still alive. The creature sneaks up on the two preoccupied men and kills them before once again heading into the water with Helen in tow.
The search intensifies around Point Diego after police find the dead mens bodies. Hundreds of armed police begin scouring the area. Ferguson briefs the police on what to expect. He stresses the sheer strength of the creature and its apparent high level of intelligence
The creature has come back ashore, however having spent extended periods out of the water it begins to weaken. Once again abandoning Helen on a deserted beach it returns to the water to regain its strength. Searchers arrive and see Helen as the creature re-emerge from the water, hearing the reported sighting police converging on the scene.
The creature retrieves Helen and heads for the water as Ferguson arrives, using the conditioning training taught at the aquarium he makes the creature drop Helen. The police seize the opportunity and open fire with a volley of gun fire
Helen and Ferguson embrace as we see a final shot of the creature drifting lifeless towards the ocean floor