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Adams.......................Leslie Nielsen
Crewman.....................Pat Culliton
Radio Man...................Robert Lipton
Sonar.......................Nigel McKeand

Maybe he'll dig up a school of tuna next and we'll all go fishing!--Patterson

On a tropical island, military scientists are preparing to launch a rocket. The officer in charge begins the countdown; they have some trouble with their radar--some sort of underwater sound. They hear a rumbling noise and feel the vibration, possibly an underwater earthquake. Despite the potential problems, the countdown continues--if they stop now, they could lose months of work--until finally the rocket explodes and the walls of the control room collapse around the men inside.

The Seaview is at sea; Adams, the man in charge of the failed rocket launch, steps into the observation nose where Nelson and Crane are at work; he complains that the ship has passed the coordinates where he had requested they submerge. Nelson tells him that Crane wanted to scan the surface with radar first. Adams says that he hopes next time they'll let him know before changing his plans, and leaves. "I hope I can remember he's considered a genius," says Crane. Nelson replies that Adams is under a lot of strain; twenty men were killed on that island and he's responsible. The radar man notifies Crane that the surface search was negative, and Crane orders the ship to dive.

Shortly, Crane and Morton are in the control room when Adams calls, telling them that he's reading underwater noise on his hydroponic gear in the lab, and orders them to set course for 215. Crane asks what depth he wants, but Adams doesn't answer; he tells Morton to set the course and leaves.

Crane walks toward the laboratory, and meets Nelson by the door, which has a sign reading "Do Not Enter." Crane asks if Nelson heard Adams' orders; he thought they were looking for a source of ultrasonic sound at 4000 feet. Nelson replies that they are--the theory is that someone has planted a device there to interfere with the missile program. In that case, asks Crane, why are they changing course? Nelson says that Adams doesn't believe that theory--he thinks there may be a natural cause, and they're to give him full cooperation. They enter the lab.

Adams greets Nelson heartily, Crane less enthusiastically; he says he has found the source of the sound--a herd of porpoises. He shows them a device that shows audible sound and another that shows ultrasonic sonar calls. Crane comments on the fact that they are following porpoises; Adams says that he knows, and tells him to continue on that bearing. When Crane asks for how long, Adams says he intends to investigate every source of sound in this area that could enter the ultrasonic range. "Yes, Captain," says Crane, exchanging a look with Nelson and leaving. After he's gone, Nelson says to Adams that tracking porpoises seems to be a waste of time; Adams says that they've tracked them long enough to know that they aren't the source in question. Nelson asks why he didn't just tell that to Crane--after all, he's in command of the ship. "As a bus driver is in command of a bus," responds Adams. They discuss the mission further, and Nelson says that there are two possible sources, a manmade one or a natural one; "Your logic is impeccable, Admiral," says Adams drily. Nelson tells him that they are there to help, and within reason they'll aid in any way they can.

In the mess hall, Kowalski joins Patterson and another crewman at a table for coffee, laughing about Adams' porpoise hunt. The crewman says that twenty men died working on the missile project--he heard that Adams "pulled a real goof." Kowalski say that he heard Adams almost got courtmartialed, and maybe he's trying to cover it up with this porpoise nonsense. Patterson suggests that Adams may find a school of tuna next, so they can all go fishing! They all laugh. Over the ship's radio, Crane notifies Nelson that they've lost the porpoises--the group broke up and scattered.

Nelson goes to the control room to find out their position; they're at their original coordinates, and there are 4000 feet of water beneath them. The panel behind them starts to flash, indicating that sonar is picking up high frequencies--it's ultrasonic sound from a stable source, causing the same sort of radio interference that Adams experienced at the missile launch. It also interferes with the Seaview's cameras, and they are now blind. Adams says he'll go outside to investigate, and Crane says he'll accompany him; "If you wish" shrugs Adams, and leaves. Crane sighs, and Nelson tells him to be patient.

In the mess hall, the men hear a hum, rising in pitch until glasses start to explode. The men go to the table to look as Crane enters and tells them the sound is causing it. The hum rises again and the glass on a gauge shatters.

In the missile room, Crane and Adams get into scuba gear. Crane asks Adams whether this sonic barrage is like the one they experienced during the missile launch; when Adams replies that it is, Crane asks why--since they must have known that something was wrong--they didn't halt the countdown. Adams, angry, says that the sonic barrage would have caused the missile to explode in any case--his decision to continue the countdown had nothing to do with it. They finish getting ready in silence, Adams giving Crane a few dirty looks as they do so, and then leave the ship.

Outside, Adams notes that the audible part of the sound is much louder out there. Crane says that it's starting to get painful. Adams clutches his head in agony; they quickly return to the ship.

Soon, in the lab, Adams and Nelson look at the gauge--the ultrasonic sound is up to 20,000 cycles! That could certainly be the sound that destroyed the missile; now they have to find out what it is. Crane enters, saying that they are on standby for emergency ascent. Adams says that they can't leave now, looking to Nelson for support; Nelson says that they won't unless it's absoultely necessary.

Nelson, Adams and Crane run to the observation nose and look out the window to see the source of the sound--a giant devilfish (manta ray), almost the size of the Seaview. Nelson and Crane look out in awe and horror as it rams the Seaview, throwing them forward. Crane orders the ship to surface immediately, but Nelson insists that they stay. Crane can't believe that Nelson is willing to risk the ship like this, but Nelson says that they have a job to do. Adams arrives, asking what happened, and sees the giant; Crane says that they are clearing out. Adams does a double take--they're running away? No, says Nelson; he tells Crane to take all possible precautions, but they are staying until they have some answers. After all, says Adams, what harm has it done? "None--yet!" replies Crane. Nelson says there's no time to argue--they need facts. The manta swims away, to Crane's obvious relief.

In the lab, Adams and Nelson listen to the taped sounds made by the devilfish; Nelson, reading, says that it's not unheard of for mantas to grow to be very large, up to 20 feet and 3000 pounds. Adams says that the fish that attacked them was ten times that size; he also wonders about the attack, since mantas are sedentary creatures. Nelson points out that the manta is related to the shark, and Adams agrees that this could account for the ray's behavior, but not for the sound. Nelson says that there's no record of any manta ray having even the potential to produce such sounds. Adams wonders if it's some new species. Crane enters, saying that since the first attack, the manta has ignored the Seaview. Nelson says that's just as well, but Crane wonders how long it will continue to do so. Morton calls, saying that the manta has slowed and is settling on the sea floor; Crane says he'll be right there, and all three leave.

In the control room, they watch the manta on the viewscreen. Adams says that it's definitely the source of the sound--after all, bats use sonar, as do porpoises. Why not? says Nelson; it would be useful for finding food, or as a defense mechanism. Yes! says Adams; perhaps they can find out how much power it can develop. Nelson suggests that it may be able to develop enough power to immobilize the Seaview. Crane says to activate the ship's sonar, and an interference pattern appears on the screen. The pitch rises, and Nelson says that the manta may sense the ship's sonar, react to it, and fight back; Crane says to cut the sonar, and the viewscreen clears. They see another manta in the water, a smaller one. To Crane, Adams says "If you want to come along, I'm going to try to capture the smaller one." Nelson and Crane exchange a look; Crane says he'll take the chance, and they leave.

Crane and Adams exit the Seaview, armed with spearguns. Soon they find the small manta and shoot it with a tranquilizer, then haul it aboard the ship.

In the lab, the manta regains consciousness and starts to emit ultrasonic sound. They look at the device measuring the sound made by the larger one, and it appears that the giant is responding to the cries of the smaller manta. Adams says that the two are obviously in communication, but Nelson is unconvinced and says they need further evidence before their investigation is over; Adams, however, is sure they are on the right track--and if the giant manta is what killed the men on the island, it must be destroyed! Nelson says that they have a lot of work to do before they can prove that; "A long way to go," says Adams, "I wonder how much you'd shorten that way if you'd been in command when those men were killed."

In the control room, Crane and Morton look at the viewscreen. Morton says that the crew is starting to be bothered by the creature, and Crane says that he is, too. Morton says that the smaller one seems harmless enough, but Crane wonders whether it can signal the large one. Morton comments on the way the giant is moving; Crane takes a closer look and concludes that the small one has signalled it, and says he wants the Seaview ready to move out at full speed at any minute.

In the lab, the small manta floats in a tank. Nelson calls Crane, ordering every nuclear warhead on the ship disarmed immediately. Adams paces, turns on the measuring device, and watches it. Soon the large manta appears and rams the Seaview. Nelson tells Adams to get away from the controls; the manta rams the sub again, and Nelson shuts the machine down. He goes to a pipe, noting that it leads to the ballast tanks and should be cold--instead it's hot. He accuses Adams of risking the Seaview; Adams replies that he did so to make a point. Nelson says that if he does anything like that again, he'll end up in the brig.

In the control room, Crane is ordering the buoyancy tanks pumped out one at a time and refilled ith cold water--the steam is to be vented from emergency valves. Nelson enters the room, and Crane informs him of the problem--something is heating the salt water in the tanks. Nelson tells him that the sound waves are doing it. Again, Crane insists that they leave; "We must balance the risk against the knowledge that might be gained," responds Nelson, saying that the last incident was Adams' fault, and won't happen again. Crane says that they are making the same mistakes that Adams made on the island, ignoring the danger signals. Nelson replies that he promised Washington that they would see this mission through. Adams comes into the room, hears the conversation, and again says that they cannot leave; he apologizes for his earlier actions and tells Nelson that since his judgment is under question, he'll let Nelson take over the mission. Nelson agrees, and Crane asks if that means they are staying here; he and Adams begin to argue, with Adams saying that Crane is unwilling to take any risks, and Crane replying that Adams takes foolish risks. Nelson interrupts, telling Adams not to interfere with Crane in any way, and leaves. Adams and Crane glare at each other, and Adams follows Nelson out of the room.

In the observation nose, Nelson tells Adams to hold his tongue in the future--he has enough problems. Adams asks what Nelson's orders are, and Nelson says they must find out the strength and range of the creature's ultrasonics. Adams says that in that case, they'll have to stimulate it to attack. Nelson says that they'll do nothing of the sort, and Adams asks what he suggests--how else will they make it use its ultrasonic weapon? Nelson thinks for a moment, then says they'll follow it. Eventually it will have to find food. Adams says that that may take some time, and he hopes they can all be patient; Nelson replies "You'd better be."

In the lab, the small manta is still sending out its signals. Adams enters quickly and amplifies them; of course the large manta reacts, and its strong ultrasonics affect the Seaview's wiring as panels in the control room begin to smoke. Before anyone can respond to that, the manta rams the sub once again. Nelson tells Crane to take the ship up, but before he can, the manta attacks from above and knocks them down.

Nelson and Crane sit discussing the situation. Crane asks damage control to report; the electrical system is damaged, and there has been some flooding. Crane says they must establish manual control as soon as possible, and surface. Nelson says that the attacks should stop as soon as the small manta is gone.

Morton, Kowalski, and a crewman enter the lab. Adams points out that the sign on the door reads "Keep out," but Morton replies that they are following Crane's orders and tells Kowalski to proceed. Kowalski opens the door to the room where the small manta is being kept. Adams tells him to get away, but Kowalski replies that it's the captain's orders and goes in. "Captain's orders---hmmph!" says Adams, and leaves.

In the missile room, Kowalski puts the small manta in an airlock to release it to the sea. Crane and Adams enters, and Crane tells Kowalski to continue. Adams, incensed, says "Crane, I'm warning you--you'd better listen to me!" Crane says that they've listened to Adams too much already. Adams tries to stop Kowalski, and they struggle until Adams is pulled away. Adams calms himself with some effort, saying "This was your idea, wasn't it?" Crane replies that it was Nelson's idea, and his order. Adams asks about their mission to investigate; Crane says that evidence will do them no good if they are all killed. "We'll see," says Adams, and leaves. Crane tells Morton to try to get underway as soon as possible. He calls engineering and is told that the main problem is now in air recirculation--too little oxygen, too much carbon dioxide.

Adams storms down the corridor to Nelson, demanding to know why the small manta was released. Nelson asks if he beamed its signal outside the ship, and Adams denies it, but asks how Nelson can insist on more facts and then remove such a crucial source. Nelson says that the situation has changed with the damage to the Seaview, and as soon as they can move he'll be ordering a salvo of torpedoes into the creature. "But what if I don't have enough information to be exonerated?" asks Adams, saying that he needs more time. Nelson replies that they are dead in the water, existing on emergency oxygen--is Adams so far gone that he can't understand? Adams leaves without answering.

In the control room, Crane orders the crew to try to take the ship up, and the Seaview starts to rise.

Crane and Nelson debate their next course of action. Crane asks about Adams, and Nelson says that he must know they are underway by now, and he ought to know why.

In the lab, Adams turns on the taped ultrasonic sounds they had acquired earlier and starts to broadcast them. He barricades the door, and waits.

In the observation nose, Crane and Nelson notice that the large manta is following them. Nelson wonders why, then realizes that it must be Adams' doing. He and Crane go to the lab and demand that Adams open the door. There is no answer, and Crane calls for emergency rigging, for men with torches to cut through the wall, and for electricians. Nelson tries to reason with Adams, whose reply is that caution is not one of his virtues. Sonar reports that the manta is approaching, and Crane orders evasive action and requests more speed from the engine room, telling the crew there to go ahead and overload the engines if necessary. Morton reports that the hull temperature is up to 85 degrees; Crane orders the air conditioning to be put on emergency full. The manta swims next to the ship.

The crewmen start to cut through the laboratory wall as Crane and Nelson watch; Crane notes the heat, and Nelson says that they are better off submerged. Morton says that the temperature is up to 95 degrees now. Crane says that if they surface they can put the crew in life rafts, and orders it; Nelson says he just hopes they can hold together that long.

The manta rams the ship, this time affecting the reactor and causing the ship to lose power. Crane orders a neutral buoyancy established. The Seaview lands on the ocean floor, and with the manta hovering over them they can't surface. In the control room, panels explode and crewmen are injured.

In the lab, Adams has stripped to his t-shirt because of the heat. The crewmen continue to cut through the wall, but it is slow work. One of the men collapses, and Nelson orders him taken to sickbay.

The manta remains in position over the ship. In the lab, Adams works frantically, bringing the sound to an even higher pitch as he hears the crewmen trying to get through the wall to stop him. Meanwhile, Nelson has an idea; he may be able to devise a sonic beam to drive the manta away long enough to make repairs. He goes to the control room to work on it.

The manta rams the ship once more; another of the torch-wielding crewmen collapses from the heat and is replaced. Crane calls Nelson in the control room--it will be another 10 minutes before the men get through. Nelson tests his beam and it works--the hull temperature starts to drop.

The men are nearly done cutting through the wall. Crane calls Adams on the radio, but he still refuses to open the door; he asks Kowalski for his pistol, kicks through the cut panel, enters, and unbars the door, letting Nelson and the others in. Adams triumphantly announces that he has gathered sufficient data to clear his name--now they'll have to listen! Nelson says that no one will ever listen to him again--what he's done is mutiny, and if he has his way Adams will be in jail. Adams can't believe what he is hearing, saying that they are all against him. Nelson tells Kowalski to bring Adams to the brig. Adams begs Nelson to at least look at his notes, and as he gives them to him he pushes Nelson into Kowalski and runs off. Nelson and Crane follow, but Adams enters the missile room, bars the door and prepares to exit the ship.

Outside the missile room, Nelson tells Kowalski to go in through a ventilation duct. Nelson shouts through the door to Adams, but Adams refuses to open it, insisting that he is going to kill the creature. Nelson asks Crane how long until they can raise the ship; Crane says that they can do so at any time, but the manta is still above the ship--however, if they can raise the ship they can destroy the manta with torpedoes. Nelson tells this to Adams, but Adams doesn't trust him to do it, saying that he has to take care of this himself. They hear him leave, and once in the missile room, get into scuba gear themselves, saying that they can at least try to save Adams.

Outside the ship, Adams plays the taped ultrasonics to attract the manta. Crane and Nelson come up behind him, but he breaks free, swimming toward it. Crane and Nelson fire spearguns with explosives at the manta; the explosives hit it and it is killed. They haul Adams back to the ship.

Back on the Seaview, Morton says that Adams is in sick bay, under sedation, and should be all right physically. Nelson feels sorry for him--all the guilt he's carrying. They see the small manta following them, and Nelson thinks they may be able to continue their research after all. Crane wonders if it thinks the Seaview is its mother, and Nelson laughs, saying that it's the only moving thing around that's big enough!