Culp and Blackmer head down a river via a boat, and head towards a large house. As they go, they see a few alligators moving around. Blackmer says he'd love to take a pot shot at one of these brutes. Culp says he wouldn't recommend it, as it might stir himself up a whole heap of trouble. The boat continues to head through the river, and isn't too far away from the house.

Inside, Mrs. Files sees the boat. She reports to Dr. Orchard and says the visitors are arriving. Orchard and his assistant McGill go to meet them. Orchard introduces himself and McGill, but Blackmer says he'd appreciate it if they could get this business over with as soon as possible. He tells Culp to wait here.

Culp says he'll wait here, otherwise he'd never get back to the river station. Orchard says they don't have to take any insolence from him, he gets paid to run that boat of his up this stretch of river and that's it. They then hear the sound of thunder, and see some dark clouds overhead. Blackmer says he doesn't want to get stranded here in a storm, and wants to get this thing over with.

Blackmer, Orchard and McGill head into the house, and go down to the laboratory. Orchard says what they're about to disclose him him has been shown to no one outside the four walls of this laboratory. Blackmer says to just give him the general picture, so Orchard tells him to take a look at the plants in the cabinet over there. He says that those are the sidonicus americanus, a very rare plant that grows only in this part of the river. It has to be processed and gathered in this lab within hours of being picked, but it has astounding properties. Outside, Culp secretly listens in to the conversation.

Orchard says that the world is heading for a grave food shortage, but this plant is going to solve that problem. Blackmer asks if they're going to start eating plants. Orchard says not quite, but tells him to look at that beaker over there. The processed juices it's collecting can be used as an additive to the diet of the livestock of the world, and could develop animals that would enlarge their meat output fantastically. McGill says this is called theramine.

Orchard says their experiments have proved it. He tells McGill to put the theramine away for the day, so he does, but Culp sees which cabinet he's put it in. The group then enter another room, which has several cages in it. Orchard says that these rabbits were imported from North America last year.

On one side, the rabbits have been fed and cared for with a normal diet. But the rabbits on the other side have been given a minimal dose of theramine, and are much larger as a result. Blackmer thinks that's some lettuce they've been fed. McGill says these rabbits have been only been given one dose of the drug, but have otherwise been fed normally. Orchard asks Blackmer if their new discovery has commercial possibilities. Blackmer says absolutely, and thinks it's a most remarkable achievement.

At Tracy Island, Thunderbird 2 takes off with Virgil and Alan inside it. The launch is watched by Jeff and Grandma, and Grandma asks if this is going to be dangerous. Jeff says of course not, there's nothing to worry about. Alan has an excellent head for heights, and he's just going to fix the aerial mast. As they head back inside, Thunderbird 2 approaches the mast.

Alan is lowered onto the mast via some cables, which he then detaches. Virgil tells his father than he's gotten Alan up there. Jeff bets it must be pretty windy up there.

Alan opens up a control panel and says he's located the fault. Jeff asks what the problem was. Alan says some of the junction plates on the leads from dish number 7 have got corroded. Jeff asks how long it'll take to fix, so Alan tells him no time. He says to have Virgil make one trip around the island, and he'll be ready to be picked up. Just then, a strong gust of wind nearly blows Alan off the mast.

He manages to hang on and says that was a close one. Jeff says this is where they find out if the fault is really down here on Earth. He tries to contact Thunderbird 5, but John's portrait doesn't light up. All he and Grandma can hear is the sound of static. Grandma thinks this means they're out of touch with John.

Jeff says their whole organization is thrown out of action until they can get through to John in the space station. But then, John's portrait lights up. John gets through and says he's glad to be back in touch. Jeff explains about the corrosion on the junction plates on one of the dishes with the beacon, and Alan must have just fixed it. He thinks it's good to know they're back in business again.

At the house, the storm has gotten much worse. Files asks if that will be all, and confirms that everything has been locked up. She bids them goodnight and then leaves the room. Blackmer thinks it's mighty good of them to put him up for the night like this. Orchard says he'd never get back to the river station in a storm like this. Blackmer asks about Culp, and Orchard says Files is going to fix him up on the couch in the study. He thinks that Files knows more about Culp's past than she admits to.

McGill says Culp was an associate of the previous owner, Lopez, and that Files was the housekeeper then too. Blackmer says he doesn't know how he and Orchard have stood it for so long in this God-forsaken spot. The lights then dim and get brighter a few times, but Orchard says that's always happening. The door then opens, so McGill gets up to go and have a look in the hallway.

He sees Files standing at the top of the stairs. He says he thought she'd turned in for the night. She says she thought he may be wanting the keys to the laboratory before the morning, but he says they won't be going into the lab tonight. He tells her to look after them as usual. As they leave, they are unaware that Culp has been hiding behind a grandfather clock, and has heard their conversation.

Later on, everyone has gone to bed. Everyone apart from Culp, who sneaks into Files' room and takes the keys from her desk. He immediately heads down to the lab and enters it, where he heads over to the cabinet that McGill had placed the theramine into.

He takes the beaker out and pours a sample of it into a vial. But as he places the theramine beaker back down, it tips over into a sink. The theramine spills out and goes down the drain.

He rinses the sink out with water. He then notices another beaker full of green liquid, so he uses it to fill up the theramine beaker again. He then places the theramine beaker back inside the cabinet and closes it.

Meanwhile, the spilt theramine is flushed out of a drain pipe, where it enters the river. Some alligators happen to swim through it. The next morning, Blackmer is ready to leave and is on Culp's boat. Blackmer thanks Orchard for the hospitality, and says he'll be seeing him. He doesn't regret his journey now, and is going to get this theramine on the market as soon as possible. He'll be sending Muller, his top research man, down to see him right away.

Back in the lab, McGill takes a look at the theramine beaker and can tell straight away that it has been tampered with -it's not theramine! Back outside, Blackmer and Culp leave on the boat. But as it moves down the river, a giant alligator sees it. The alligator enters the water, and another one quickly follows it. They start heading after the boat.

The boat suddenly rocks. Blackmer tells Culp to take it easy and quit rocking the boat. Culp says it wasn't him, but then starts screaming when he sees a giant alligator right next to his boat. Back in the lab, Orchard says he thought this laboratory was locked up as usual last night. Files says it was locked, but McGill says nevertheless, someone has been in here overnight. Orchard wonders if Blackmer or Culp could have been in here, but McGill thinks this is terrible -that drug could have horrible consequences if it got into the wrong hands. Then then hear the sound of gunshots.

Blackmer has started shooting at the alligator, as Culp screams at him to kill it. The others head outside the house and see the giant alligator. Orchard says the theramine has gotten into the swamp, and says those men don't stand a chance. They then see the alligator hit the boat with so much force that it turns over.

Culp and Blackmer and thrown into the river. The alligators retreat and Blackmer resurfaces. McGill uses another boat to head out to where Blackmer is. He pulls him back into the boat and starts to head back towards the house with him.

The two alligators then enter the river again, and start chasing after the boat. McGill speeds up and grounds the boat near the house. But more alligators then appear and head towards Orchard.

He runs back to the house, and once he gets back in, Files slams the door shut. She says the others are alright, as they got in through the side entrance. Outside, an alligator starts pacing around the house, but then starts smashing a wall with its tail. Orchard wonders where can they go, as those brutes will have the house down in no time. Files says they should go to the laboratory, as that's the safest place.

Everyone goes to the lab. McGill says they've got to get help, and is going to radio the river station. Blackmer asks what the heck are they going to do, send out the militia? It'd take them hours to get here. They then hear the alligators roaring outside, and Orchard says this house isn't going to be able to stand up to this punishment. He asks what they're going to do, so Files suggests sending for International Rescue. Orchard says if anyone can get them out of here, it'd be International Rescue. McGill makes the call.

John picks up the call, so he contacts Tracy Island. Everyone is surprised at the idea of giant alligators. John says the place is in a lonely spot far up the river, so it might be tricky to land. Jeff tells him to tell them International Rescue are on the way. He then tells Scott to go. He asks Brains what he makes of this, and Brains says they really don't have enough to go on yet. It sounds like it's some freak mutation, and he hopes it's localized.

Scott takes off in Thunderbird 1. Jeff says that conventional weapons are out of the question. Virgil says with those four people in the house, they might not be able to get too close to the alligators for selective fire. Jeff thinks pod 6 would be the best for this operation. Virgil, Alan and Gordon blast off in Thunderbird 2.

Jeff and Brains look at a map. Brains says it's pretty swampy terrain, so this is going to be a difficult one. Jeff wonders how those people are standing up to the alligators. In the house, Orchard wonders where International Rescue are. McGill tells him they're on the way, so he should relax. Orchard asks how he's supposed to relax when there are three giant alligators tearing the house down! Outside, one of the alligators smashes part of the house with its tail, causing Orchard to fall over.

The people in the house then hear the sound of jet engines. They realize International Rescue have arrived, as Thunderbird 1 flies by the house. Scott reports that this looks pretty hazardous, as there are three massive alligators attacking the house where these people are. He thinks the alligators look like sturdy brutes, so he doesn't know how effective their missiles are going to be. And besides that, the house is right in the line of fire. He then tries to contact McGill.

McGill responds and says his group are in the basement laboratory. He asks Scott if he thinks he can get them out in time. Scott says he's going to try and frighten these reptiles off with a slight diversion. His buddies are on the way in another craft, so they will get them out of there. Scott contacts Thunderbird 2 and explains the situation. He says he's going to try and clear the area, then get in the house. Virgil says he'll be over the rescue zone soon.

Scott finds a clear area to land Thunderbird 1. He heads out on his hover bike and tells McGill he's nearly there. He wants everyone to stay well clear of the lab window, as he'll enter the house that way. Scott then stops his hover bike and fires a missile at the house, scaring some of the alligators.

The alligators retreat back towards the river, so Scott rides up to the house and enters the lab. He asks if there's no other way out of this house. Files says there used to be one other way, and explains that the previous owner, Mr. Lopez, had a secret landing stage built for emergencies with an underground passageway from the house. Orchard asks where this passageway is.

Files says Lopez never told her where it is, he only told one other: Culp. They then hear some banging noises and see that the alligators have returned. One of them starts smashing its tail on the side of the house. Scott tells everyone they've got to get out of here, so they leave the lab. Debris soon hits the lab, smashing up the equipment inside it.

An alligator does so much damage to a wall that the wall breaks apart. Scott and the others take cover in the hallway, and McGill thinks the alligators seem to be all round the house now. But Files then notices a bookcase start shaking. McGill thinks the alligators have got in, so Scott tells everyone to stand back.

But when the bookcase moves, it's Culp who was behind it. He shoots Scott's gun out of his hand and McGill says the alligators didn't get him after all. Culp says that's a brilliant deduction, and says that he used Lopez's secret waterway. But he says this is no time for explanations, they're going to do things his way, or he can cause even more unpleasantness with Dr. Orchard's remarkable discovery.

Virgil contacts base and says he doesn't get it. Scott got into the house alright, but for the last few minutes his line has been dead, he just can't get in contact with him. Jeff thinks Scott must have some scheme in hand. He asks Virgil how far he is now from the danger zone. Virgil says he's arrived, and says the first thing they've got to do is get those reptiles away from the house. He flies Thunderbird 2 over the house and uses its jets to scare the alligators away. Gordon tells Alan they'd better standby with the tranquillizer guns.

Culp says the alligators are moving back to the river. Virgil wonders what's going on in that house, and tries to contact Scott. Scott goes to answer his radio, but Culp tells him if he just says one word, it'll be his last. Virgil keeps trying to get through, but gets no answer.

Alan tells Virgil that he and Gordon are all set. The two use the tranquillizer guns and take out two of the alligators, who roll over and go to sleep. Virgil flies Thunderbird 2 back over and they see that the alligators have been hit alright, but Gordon asks about the third one. Virgil says it's heading back to the house, and is already resuming smashing up the place.

Virgil lands Thunderbird 2. Gordon says they just can't use the tranquillizer guns so near the folks in the house. Alan says he's got it: he'll go out on his hover bike and draw the alligator off. Virgil thinks he wouldn't stand a chance, as the hover bike cannon would be useless against the alligator. Alan says they've got to do something. Back in the house, Scott tells Culp than in just about two minutes from now that wall will collapse. He just wants to use the ray gun so he could maybe cripple the reptile.

Culp tells him one move and he's dead. Meanwhile, Alan heads out of Thunderbird 2 on his hover bike. He quickly gets the attention of the remaining alligator, so he flies away from the house. The alligator starts going after him.

Alan lures the alligator away from the house, telling the reptile to come and get him. The alligator continues to follow Alan, but seems to get cautious. Alan slows down and speeds up again, causing the alligator to resume giving chase. But due to not watching where he's going, Alan then crashes his hover bike and falls off it.

He hits his head and can't seem to get back up, as the alligator moves towards him. Gordon then fires another shot from the tranquillizer gun. The shot hits the alligator and takes it out.

Culp says the coast is clear. He intends to be the first on the market with this little potion, so he tells Scott to radio his buddies and tell them he's coming out now. Scott says it's not safe yet, and Orchard says they don't know how many more of these monsters are out there. Culp warns them if they don't do as he says, there's going to be more of them. He threatens to pour his theramine vial into the river if anyone tries to stop him. Scott calls Virgil and explains things. Virgil tells Gordon to standby in Thunderbird 4.

Virgil takes off in Thunderbird 2, but leaves pod 4 behind. The pod door opens and Gordon launches Thunderbird 4 into the river. Meanwhile, Culp gets to the remaining boat, and uses it to head up the river.

But another alligator sees him and gives chase. It attacks his boat and causes him to fall out into the waters, where the alligator drags him under. Orchard wonders what's happened to the vial, and Scott says he hates to think about it.

Virgil fires a missile, which kills the alligator. He tells Gordon that they've got to find that vial. Gordon switches Thunderbird 4's lights on and happens to see the vial sinking nearby.

Orchard says if that liquid gets mixed into the river, this nightmare will start all over again. Scott asks Virgil if there's any hope of finding that vial, and Virgil says Gordon is going out to have a look. Gordon leaves Thunderbird 4, and is eventually able to find the vial. He says he doesn't think there will be any more giant alligators.

Later on, Scott and the others have returned to Tracy Island. Jeff tells them they certainly did a great job. He says that Blackmer and Orchard have just announced that the drug is going to go under international control. Scott says he thinks that's the best thing for it. Tin-Tin then arrives back from her shopping expedition, and asks about the rescue. Virgil and Scott give unenthusiastic responses, so Tin-Tin says they can tell her about it later. She then tells Alan she has something for him, although she knows it's not his birthday until tomorrow. She tells him to come and see what she's brought back for him, it's in the bathroom. As Alan walks off, Scott wonders what Tin-Tin wants to show Alan in the bathroom.

Alan opens his eyes and sees something in the bathtub: a baby alligator! Tin-Tin recalls him saying how nice it would be to have a pet. Alan says it's a baby alligator, but Tin-Tin says this one's fully grown, as it's a special breed of pygmy alligator. Alan thanks Tin-Tin, as this is what he's always wanted -a cute little alligator!