Strange goings on at the site of an experimental underwater laboratory as a mysterious object of unknown origin is detected in the area. A group of researchers investigate and come face to face with the object, a large automated extraterrestrial probe. They board the craft and discover a small object at first considered to be some sort of instrument package, which they take back to the lab for closer inspection. It is then that events take a turn for the worse.
A speedboat arrives at a navy research ship anchored close to a coral reef. Commander Wayne (Scott Brady) boards the ship and is introduced to Skipper (Roy Barcroft) . Skipper lets Wayne know something strange has been found and no one quite knows what to make of it. Skipper arranges for Wayne to go down to Sealab, an underwater research facility anchored to the bottom.Arriving at the installation he is greeted by Dr James (John Howard) who is in charge of the medical facilities. Dr Rene Peron (Sheree North) who is a marine biologist, then finally Dr LasSatier (Gary Merrill) he is tracking an unknown object about a mile and a half from the installation. Listening to the sonar, Wayne decides there is no known propulsion moving the object. He also dismisses the possibility it is a whale.LaSatier then introduces Wayne to Dr Wilson (Biff Elliot) who is the teams electronics expert. Wilson explains the object is making noise in the ultra sonic level, well beyond human hearing. Renee then reports the object is now in range of the mini sub piloted by Hugh Maddox (Mike Road). Maddox who is the teams best diver has some history with Wayne, who seems concerned he is involved with the mission. Maddox reports the object is huge and probably a submarine. Before they can get closer the unknown vessel moves off.Sandra Wells (Wende Wagner) is co pilot of the sub. She is critical of Maddox impatient actions; if they had taken extra air tanks they might have tracked the object longer. Lasatier then alerts Maddox to the presence of Wayne on board. He seems equally disgusted the navy has sent him of all men.The photos taken by Maddox reveal little detail. Wayne wants to arrange a combat unit to patrol the area. He is concerned the craft is some new development by the Russians, and he needs to learn as much as possible about itA report comes through from the control room the object has reappeared and headed towards the installation on an attack course, Commander Wayne thinks there is no threat, the unknown vessel has had plenty of opportunity to attack in the past and didnt The unknown vessel does a very close pass of the installation but causes no harm. It then drops out of sight in one of the deep trenches that surround the area. The installation seems to be intact, however the radio lines connecting the station to the surface has been cut.Unseen back at Sealab, the now stationary vessel deploys a small probe into the main entry chamber of the craft. It is encased in ice and the size and shape of a small scuba air tankWayne decides it is time to go and investigate the craft. Maddox is selected to accompany Wayne on the mission. This creates tension between the two men. Maddox remembers a situation where he believes Wayne left some trapped men to drown in a submarine accident. Wayne counters with the observation Maddox was one of the men, and he survivedThe two men head out in company with Sandra Wells who will be photographing everything they encounter
After swimming around the vessel once, they find an access hatch and enter. It is a large featureless chamber. The party finds the container previously placed there by an automatic process. They are unsure what its purpose and decide to take it back to Sealab for further study.Making measurements of the retrieved probe they detect a signal being transmitted at similar frequency to those detected on the main ship. Some of the party wants to cut the probe open and see what is inside. Wayne vetos the plan, he is concerned what might be inside and wants to wait for the Navy to issue instructionsPeron goes back to studying the object and believes it is larger, as if it is growing or expanding. Re-measuring the item they find it is 2 inches longer. They try to determine what is feeding the device. Dr LaSatier thinks it might be absorbing heat.A high pitched squeal begins to echo through the lab. Rene screams as she sees the object is doubled in size. Dr Wilson remains behind in the lab to secure some fragile equipment. The sound increases in intensity and Dr James grabs his head causing him to drop a canister of acid he was movingDr Wilson escapes the lab but collapses, the others realize the crisis and cut the ventilation system to stop the spread of acidic vapor. Cmd Wayne and Tex (William Thourlby) enter the lab to get things under control, they see the probe has broken open, but before they can understand what this means a humanoid creature attacks them. Cumming escapes and braces the hatch. Tex apparently has been killed.Maddox arrives on the scene and is apprised of the situation by Wayne. Maddox reacts badly and blames Wayne for leaving Tex behind. He wants to reenter the lab but cant because the creature has broken out flooding the chamberOn the surface they realize the lines have been cut to Sealab. As they assess the situation the creature from the probe surfaces. Commander Wayne gets through to the ship just a scream comes from the deck as one of the crew is thrown overboard by the creature.Skipper goes out to investigate but is killed, finally the radio operator still trying to complete Waynes message is also killedAs the radio goes dead and Wayne thinks the creature is trying to isolate the installation so the world cant know of its existence. The creature returns to the lab and tries break in through one of the windows but failed.
Dr James reports that Wilson has a critical head injury and needs to be hospitalized urgently.Wayne explains that wont be happening as the ventilation system connected to the surface fails. James reports they may have as little as 12 hours air. Wayne decides to go outside and get the diving bell operational, Maddox agrees to help him. As they enter the diving chamber the creature surfaces and attacks.Wayne escapes but Mike (Glen Sipes) is wounded. The creature goes back into the water then sabotages a gate at the bottom of Sealab to stop any of the humans escaping. Regrouping in the control room they remaining crew weigh their options. Dr LaSatier suggests they may hold out for the supply ship arriving the next day. Wayne thinks they cant last that long. Wayne suggests closing off further compartments to try and extend the air supply. He tells James to move Wilson and the other injured men to the control room. LaSatier informs him that Wilson has just diedWayne checks on the condition of Tex, LaSatier shows him that Texs condition is worsening. Wayne is puzzled; he did not think the injuries were that bad, Dr James thinks that an unknown infection has set in. They wonder the creature may know it is infected and escaping his home planet. James prepares a vaccine and inject Wayne. He is really not sure if it will do any goodWayne decides to set a trap for the creature using the spear guns on the lab. He asks Maddox to help, who asks are you looking for more sacrifices. Wayne has had enough and decides to have it out about the circumstances of the sub years before. Wayne wants to know how Maddox got out but the others drowned. He challenges Maddox that it was him who actually killed the men. There was an escape pod they all could have used, yet Maddox was the only one who didMaddox counters that yes he did get to the chamber first, but the others would not follow, they were still at the hatch Wayne closed. Maddox said he had always had nightmare about the faces of the drowning sailors. Wayne argues he could not know that unless they were trying to get into the chamber and not through the hatch. Wayne then wants to know why two of the drowned sailors were still hanging onto the escape chamber mechanism when the sunken wreck was brought to the surfaceMaddox cracks and admits he froze when he made it to the chamber and yes he was the person who killed those men. It was the reason for leaving the service, he hated the thought he was a coward and could not look the surviving sailors in the face.Two hours later the men have rigged up all the available spear guns with trip wires. They want to lure the creature into the station and try and kill him with the spears. After a number of false starts the creature takes the bait and hit by a number of spears. It appears not to be killed but definitely woundedWayne and Ellis (Richard Niles) go into the water after the creature. Maddox watches indecisively then seems to reach a decision and go down to aid Wayne. After playing cat and mouse amid the corals, the creature finally sees a chance and attacks Wayne, the other two men join in the battle but the creature is obviously extremely powerfulAfter a lengthy fight the creature, wounded and exhausted is captured and taken back on Sealab. Wilson pumps the creature full of sedatives to knock the creature out. Wayne and Maddox then go to the surface vessel to restart the pumps and report in to the Navy about what is going on.Wayne finds all the radio equipment destroyed. Maddox get the pumps working but the diving bell and winches have been destroyed. Wayne thinks there are more of the creatures back on the original ship. He organizes all the dynamite they can find and heads of to destroy the alien craftBefore they get there a second creature is deployed by the alien ship. Dr Lasatier opposes the plan. He want every saved so the scientific community can study all the technology.Onboard Sealab the creature revives and attacks Rene and Dr Wilson, then sets out after the men heading for its ship. Arriving the team begin assembling the bomb until the creature arrives and interrupts their work
Maddox corners the creature and distracts it long enough for the other to escape.The creature stops its attack long enough to disarm the bomb, but Maddox uses a distress flare to ignite the explosives destroying the creature the ship and [email protected]