In the opening scene, McBride (Greg Evigan
) and Joyce Collins (Nancy Everhard
) are woken when their alarm clock goes off. He mumbles something then passes it off as a dream about being underwater. She recalls it is the third time he's had the same dream. They go on to discuss Captain Laidlaw, how good his life is, with a great wife and perfect children. She then asks the man why he never married. He claims he's never been in one place long enough to get to know anyone well enough to marry
Later the crew assembles in a smaller sub (DSRV2) and launch from the SEATRAK installation. The two pilots, James Richardson (Matt McCoy
) and McBride discus how dreary six months underwater have become. They pick up a location transponder which belongs to a second installation Deep Star 6 (DS6) Snyder (Miguel Ferrer
) contacts them to complain they are late; shift change was 20 minutes ago.
The sub moves in and quickly docks at DS6, John Hodges (Thom Bray
) has fallen asleep during the brief trip and has to be woken.
Overnight the on-board team had maneuvered the missile sled into position and the day crew are to begin excavation. Hodges warns the site has to sounded first to confirm there are no geological faults.
Over breakfast Capt. Laidlaw (Taurean Blacque
) insists that Scarpelli (Nia Peeples
) take the rest of the day off. She protests she has far too much work to do but the Captain orders her to stand down.
The designer of DS6 Dr. John Van Gelder (Marius Weyers
) contacts Collins by radio at the main base. She advised him that the findings look wrong, there is a cavern and the regulations say the missile sled can't be mounted there. Van Gelder tells Collins he wants the cavern collapsed and missiles installed by the end of the week.
Scarpelli is immediately interested in the cavern; she wants to do a survey of the cavern. She is aware of strange reports of marine life in the area going back 150 years. Van Gelder, while sympathetic to her desires, refuses the request.
Dr. Diane Norris (Cindy Pickett
) treats Snyder for a rash on his skin, he is concerned about the rash because the mission is running well over time and budget. He wonders if Van Gelder knows what he is really doing, and why the Navy put him in charge to begin with Scarpelli goes to see the captain about Van Gelder, and the rejection of her request. Laidlaw reinforces Van Gelder's decision. He has less than a week to get the project completed.
The charge to break open the cavern is deployed by a underwater tractor (SEACAT) driven by Hodges and Osborne (Ronn Carroll
) The detonation breaks open the cavern but causes the seafloor to subside. The tractor has to reverse at emergency speed to avoid disaster. Collins aboard SEATRAK asks the crew of the tractor to release a small remote probe to investigate the breached cavern. The probe continues down into the depths until something attacks it and snaps its guidance cable Hodges makes a decision to go and investigate what happened to the probe. He releases the control section of the tractor which becomes a mini sub. They locate the probe and begin the process of retrieving it when Osborne reports a sonar contact closing very fast. At the last second Hodges sees the object and panics over the size and speed. He begins to report an emergency to Collins when contact is lost.
Burciaga (Elya Baskin
) begins trying to locate the sub using SONAR but finds no contact at all. Collins questions how the craft could have simply disappeared. Burciaga tries various telemetry feeds but fails to find the craft. Suddenly they pick up something else, moving too fast for it to be the SEACAT. Collins contact Deep Star 6 to tell them of the loss and to warn that something else is moving around down near the base. The object suddenly changes direction and hits SEATRAK. Collins reacts to the situation by declaring a mayday. Snyder has left the control room of DS6 and her plea goes unanswered. The object repeatedly attacks SEATRAK almost pushing it over an underwater cliff before breaking contact. Onboard Collins is badly hurt and Burciaga possibly dead.
Snyder returns to the control room with the captain but there is no contact. Captain Laidlaw decides to go with McBride to SEATRAK and investigate whats happened. On SEATRAK, Collins begins moving around. She checks on Burciaga who is trapped by the legs and cant move. Collins tries to free him but he is pinned by some heavy equipment. She is concerned over the rising water from ruptures caused by the attack.
Laidlaw picks up the same SONAR contact that Collins did. Laidlaw is stunned by the speed of the object. Laidlaw orders the sub to be brought to a stop, blow ballasts and shut down the running lights. After about a minute, the object seems to lose interest in them and continues on until contact is lost.
They arrive at SEATRAK and see the damage that's been caused. Although the installation is on an angle they manage to dock cleanly. They can see the leaks flooding the control room and they detect evidence of fires on board. Working through the installation they find Collins and the trapped Burciaga in the control room. As they begin the rescue operation the installation shifts considerably threatening to collapse over the cliff at any moment. They realize Burciaga is dead and as they evacuate the installation shifts again. The movement causes a hatch to slam shut trapping Laidlaw. Laidlaw orders them to leave him. His back is broken and there is no hope of getting him out. They refuse to go, however while they manually try to reopen the hatch, Laidlaw throws a switch that floods the SEATRAC forcing Collins and McBride to leave him. Collins and McBride reach the sub and manage to un-dock just as SEATRAC lurches one more time and collapses over the side of the cliff.
Back at DS6, Collins explains the events leading up to the loss of the SEATRAK. Scarpelli thinks it might be the lights on the probe and SEATRAC that provoked the aggression from the unknown creature. Word comes through that extraction from the base has been authorized and team are to begin preparations to evacuate.
During Collins medical examination, the doctor catches the sound of a second heartbeat. Collins admits to McBride she is pregnant; McBride reacts very positively to the news and seems to indicate he may marry her. As the preparations for leaving DS6 progress, Van Gelder raises the question of what is to happen to the missiles. The base can't be abandoned till the weapons are secure. Snyder is sent to begin the procedure so the rest can leave.
However, Snyder is queried by the system computer as to the reason the missiles are to be abandoned. He enters "aggression", but the computer assumes he means "enemy aggression" and begins detonating the missiles. Van Gelder becomes angry at Snyder for detonating the weapons. He tells everyone to hang on because a force 20 G shockwave is about to hammer the installation.
The structure survives the concussion but suffers huge electronic and structural damage. Once the fires are extinguished and flooding stopped, Van Gelder informs them along with the flooding of the crew quarters; the oxygen unit has been destroyed; leaving them possibly eight hours before they run out of air and suffocate to death. The greater concern is damage to the reactor feeding power to DS6. At most they have a few hours before the reactor explodes. Finally the escape pod is unusable because of damage to the decompression system. Collins realizes they can re-route around the damage lines and get the decompression system working again. The crew set to work completing the modifications Collins recommends.
To complete the rest of the repairs, Richardson dons a deep dive suit and goes outside DS6. He completes a weld on EZ6358, as he reports his success; he realizes something is out there with him. He sends a panic message about the airlock. Assuming he is trying to return, the crew trigger the retrieval system to bring him back on board. As the suit is raised from the water separating the airlock, they try to release Richardson from the diving unit. Snyder reports something else has also entered the airlock. Before Richardson can be released, a huge crab-like sea monster (seen for the first time) emerges from the water where it reaches up and bites Richardson and the suit in half from the waist down.
As the remaining crew try to evacuate the room, the creature attacks and kills Scarpelli. Snyder, reaching safety, panics and closes the hatch while the others are trying to get out. Collins realizes what he's done and frees the others. McBride sees Snyder's actions as cowardice and attacks him before being pulled off by Collins. The survivors realize they have to fight the creature off and out of the diving room so they can try again to get the decompression system working.
Van Gelder and Collins discuss the nature of the creature. He believes from what he saw that it is an arthropod crab of some sort, only many times larger than anything ever seen. The crew finds spear guns and CO2 cartridges and pump-action shotguns. There plan is to try and hit the creature with the cartridges and make it expand till it bursts.
The reenter the flooded chamber and deploy a series of strobe lights to try and confuse the creature. As they spread out through the room they seem to be unable to locate the sea creature. Confident, McBride begins the needed repairs to the air lines. As he begins to get clear, the creature appears and the crew opens fire with shotguns. Van Gelder slips and falls backwards onto Snyder's spear causing a part of Ven Gelder's chest to expand and explode. Snyder completely panics, drops his weapon and runs to safety.
Now safe, the crew tries to deal with Snyder. He believes they are blaming him for all the deaths, even though everyone saw the Van Gelder death was not Snyder's fault. Dr. Norris gets him calm enough to give him a shot that further sedates him.
Collins, McBride, and Norris go off to prepare the decompression pod for the evacuation. Left to his devices, Snyder has a vision of Van Gelder appear and blames him for his death. Snyder, now completely insane, races to the escape pod, bypassing the decompression system and launches the rescue pod to the surface. We follow the pod's progress to the surface as at first Snyder's nose beings to bleed then lesions form on his arms and face, these gather intensity till they begin to also flow freely. Finally, Synder's body, overloaded by the changed atmospheric pressures, explodes.
In the remaining safe area of the installation the three survivors, Collins McBride, and Norris weigh up their options. They now only have three hours before the reactor blows. McBride asks Collins to marry her. She accepts. McBride realizes if he can get through the flooded section of the DS6 he can bring the remaining mini sub around to the lock with the decompression chamber and possibly save everyone.
McBride succeeds in reaching the mini sub and begins to pilot it back to the other side of the station. Collins and Norris hear a tapping sound at the locking hatch, think it is McBride and get hit by a wall of water. Finally they get the hatch closed again. Norris orders Collins into the pressure chamber, as she waits for it to open; they hear a noise in the water. Norris believes the creature is in the room with them. She slides over to a medical station and charges up a defibrillator. As the creature attacks she tries unsuccessfully to shock it. Surviving long enough for the paddles to recharge, she sticks them in the water killing her and badly injuring the creature. As Collins scrambles into the decompression chamber she finds McBride already there. They watch stunned as the creature seems to go through its death throes before slipping beneath the water.
Sometime hours later, an alarm goes off indicating decompression is complete. With only minutes to go, McBride and Collins quickly evacuate to the mini-sub and launch just as the reactor explodes. They work their way to the surface fighting through a series of shock waves thrown out by the explosion. They reach the surface and deploy the life raft. Just as they climb in, the sea creature suddenly surfaces and attacks. McBride is trapped on the sub. He releases the remaining fuel and then fires a flare gun igniting a raging inferno around the sub. After a few minutes the sub explodes, presumably killing the creature at last. Collins looks on horrified thinking he's just died. On the verge of giving up she sees him surface.
In the final shot, as the camera pans back, we see the couple in joyous embrace floating in the life raft on a calm empty sea.
Original synopsis written by Chapman_glen@yahoo.com