"I wouldn't recommend that, Mr Blackmer. You might stir yourself up a whole heap o' trouble."

In a house on the Ambro River, scientists have developed a chemical that when consumed by animals can make them grow to huge proportions. They plan to use it to end a food crisis but Culp, their boatman, has other plans. When the theramine is accidentally poured into the swamp the scientists find themselves under seige by three gigantic alligators...

Two men, industrialist Blackmer and boatman Culp, are traveling down the Ambro River by boat through a jungle. Blackmer ponders shooting some of the watching alligators for sport but Culp warns him that this is dangerous. In an old house further along the river, Mrs. Files observes their approach and reports it to scientist Dr Orchard and his assistant Hector McGill in their basement laboratory.

Meeting Blackmer at a jetty, they take him to the laboratory and show him a liquid refined from a plant, the sardonicus americanus, grown on the riverside. Unaware that Culp is observing them from the window uninvited, Orchard tells Blackmer that the plant will solve the food problems of the world. McGill locks the liquid known as theramine in a cupboard while Orchard shows Blackmer some rabbits which have been enlargened to several times their normal size thanks to a diet of theramine. Blackmer is extremely impressed but the demonstration is interrupted by the sound of a storm brewing.

Thunderbird 2 takes off with Virgil and Alan aboard and flies to the Tracy Island arial mast which needs repairing. Alan is lowered onto the top of the mast on a harness, navigating Virgil to drop him onto the platform. Alan sets to work repairing a corroded joint despite nearly being knocked off by a strong gust of wind. Soon Jeff finds himself able to contact John in Thunderbird 5 again.

Darkness has descended and the storm is raging on the Ambro. Orchard is letting Blackmer at Culp spend the night in the house. Files is distrusting of Culp, having known him when she was the housekeeper for Lopez who used to live there. McGill entrusts the keys to the laboratory to her unaware that Culp is watching.

Later on in the night, when the storm has passed, Culp steals the keys from Files' bedside. Sneaking into the lab, he takes the theramine from the cupboard and pours himself a phial of it. However, he accidentally knocks the beaker containing the rest of it over and it is poured down the sink. He takes another similarly coloured chemical and pours it into the beaker which he then returns to the cupboard. He leaves the room, unaware that the theramine that went down the plughole has now entered the swamp where the alligators are swimming.

The next morning Orchard sees off Blackmer and Culp at the jetty. Blackmer promises to send Muller, his research scientist, to visit. Meanwhile McGill finds the beaker and realizes that it does not contain theramine. Blackmer and Culp are by now halfway across the swamp and have attracted the attention of several gigantic alligators. Suddenly the boat is attacked from beneath. Blackmer tries shooting at the hostile creature and the sound of his shots alert the others. The alligator manages to capsize the boat but while Culp disappears beneath the water, Blackmer swims for the surface. McGill rushes out in his own boat to save him. They hurriedly return to the house avoiding two alligators.

A third has meanwhile sighted Orchard on the jetty and chases him into the house. Files locks the door but the alligator begins smashing it with its tail. Orchard, McGill, Blackmer and Files take refuge in the laboratory as a second animal joins the first. On Files' suggestion McGill calls International Rescue. Scott departs Tracy Island in Thunderbird 1 with Virgil, Alan and Gordon close behind him in Thunderbird 2.

Three alligators are now smashing at the walls of the house. Thunderbird 1 arrives and lands in a clearing. Scott mounts his hoverjet and fires a missile from it which hits the house, momentarily scaring the alligators away. While it is safe he enters the laboratory through the window. Files tells him that there is a secret passage from the house to a landing stage but only Lopez and Culp knew where it was. One of the alligators turns its attention to the laboratory, forcing the humans to evacuate. As they leave the roof begins to cave in.

They head for the living room but soon that is unsafe also. A bookcase slides away from the wall revealing Culp who shoots the gun from Scott's hand leaving them all at his mercy. He had escaped the alligators and used the secret passage to return to the house. He threatens to do more damage with the theramine if they do not cooperate.

Thunderbird 2 is now flying over the danger zone and Virgil is worried about Scott's radio silence. By firing the engines downwards over the alligators, Virgil is able to force them away from the house. Alan and Gordon man the harpoon guns and shoot two of the alligators who fall unconscious.

The third has meanwhile resumed its attack on the house and the boys dare not fire the guns so close to the people there. Alan plans to lure the reptile away as bait, which Virgil vetoes, but the younger brother leaves Thunderbird 2 on his hoverjet anyway. As Alan approaches the house, the alligator begins to approach him and he backs away at just the right speed to let it follow him. Suddenly while looking behind him, Alan crashes his hoverjet into a rock leaving him sprawled across the floor at the approaching monster's mercy. Quickly Gordon fires a tranquilizer, putting the creature out of action and saving Alan.

Thinking the coast is clear Culp forces Scott to give him clear passage across the swamp with Thunderbird 2 so he can be the first to sell the theramine on the market. Unknown to him, however, another previously unseen alligator is swimming in the water. Thunderbird to deposits its pod on the waterside and Gordon launches Thunderbird 4. Culp is crossing the swamp in McGill's boat when suddenly the fourth alligator attacks his boat, killing the boatman. Virgil kills the reptile with a missile.

Orchard is horrified at what could have happened to the phial of theramine Culp was carrying. Gordon leaves Thunderbird 4 and begins searching the bottom of the swamp. After a while of searching he finds it, mercifully still intact.

Back on Tracy Island Jeff announces that the theramine is now going under international control. Tin-Tin returns from shopping on the mainland with something to show Alan in the bathroom as a birthday present. It turns out to be a pet pigmy alligator.

Verdict: An all-round excellent episode. The superb jungle sets and sound effects evoke a scary atmosphere never seen before or since in Thunderbirds, particularly at night. The alligators are formidable villains and the interchanging between the real creatures and life-like puppets is almost seamless. The guest cast rank among the most well developed characters in the series thanks to plenty of exposure and their mysterious backgrounds. Some complain that the theramine is too fantastical but surely Thunderbirds can explore futuristic biotechnology as well as machinery?

Broadcast information: Broadcast on BBC 2 on Tuesday 8^th November 2000 at 6:00 pm and on Saturday 12^th November 2000.

DVD availability: On Volume 6 and the Complete Box Set.

VHS availability: On the Pod 2 Box Set and the Complete Box Set.