A business man called Blackmer travels to an isolated house via a boat driven by a guy called Culp. Blackmer meets with Dr. Orchard and his assistant, McGill, who show him a revolutionary new drug called theramine. The theramine can cause things to grow greatly in size, and Blackmer is shown a few rabbits who have been given a small dose of the drug. Believing it to be a solution to the world's food problems, Orchard hopes it will have commercial possibilities.

However Culp has been listening in to their conversation. Due to a bad storm Blackmer has to stay the night, so Culp waits until everyone is sleeping and then steals the keys from Files, the housekeeper, to gain access to the lab. He takes a sample of the theramine and stores it in a vial, but accidentally spills the rest of it into a sink. He rinses out the sink and tries to cover up his mistake by pouring some other green liquid into the beaker, before putting it back in the cabinet and leaving the lab. However, the spilt theramine enters the river, where some alligators swim through it. The next morning, Blackmer and Culp leave, but the boat is attacked by a giant alligator. Blackmer is saved by McGill, but Culp is nowhere to be seen and presumed dead.

Everyone goes to the lab, but several giant alligators get to the house and start smashing it to pieces. Files suggests they call International Rescue, so McGill makes the call. John answers it and relays the details to Tracy Island. Everyone is a bit surprised at the idea of giant alligators, but Scott is sent out in Thunderbird 1, whilst Virgil, Alan and Gordon take off in Thunderbird 2. Scott arrives first, and decides to try and enter the home. He leaves Thunderbird 1 and flies towards the house on his hover bike, and after confirming via radio that everyone is in the lab, fires a missile. The alligator retreats to the river.

Scott enters the house, but the alligators soon return and resume their attack. Files mentions there was a hidden passageway that the previous owner, Lopez, knew of, but the only other who knew about it was Culp. They then hear and see the bookcase shaking and believe an alligator is about to get in, only for it to be Culp. He pulls out a gun and reveals he still has his theramine sample, which he intends to dump into the river if he isn't given a clear path to get out of here. He intends to be first on the market with the theramine. Thunderbird 2 arrives, and a couple of alligators are taken out via Alan and Gordon shooting them with tranquilizers.

One alligator remains outside the house. Not wanting to put the people inside it at risk, Thunderbird 2 lands and Alan leads the remaining alligator away by getting it to chase after him on his hover bike. Thunderbird 4 is then launched, but Culp sees that the coast is clear. He leaves the house and uses the remaining boat to try and escape, only for another giant alligator to show up and attack him.

His boat is destroyed, and he gets thrown into the water where the alligator drags him under. Virgil fires a missile at the alligator to kill it, letting Gordon search for Culp's theramine vial. He uses Thunderbird 4's lights to find it and then leaves his craft behind. He eventually finds the vial and says that he doesn't think there will be any more giant alligators.

Scott and the others return home, where Jeff congratulates them on a job well done. Tin-Tin then returns and says she has an early birthday present for Alan, which is in the bathroom. Alan goes to see what it is, and finds out it's a baby alligator. 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!

Notes

-Whilst the episode is called Attack of the Alligators!, baby crocodiles were actually used.

-Due to problems with the alligators, this episode took longer to film than expected -it was meant to be one month, but ended up taking two.

The alligators did not want to move much at first. The crew tried to get them to move by shocking them with electrodes, but this didn't work and led to the RSPCA getting involved. An employee from the RSPCA was sent in to check on the production crew, but when he found out that they were filming Thunderbirds, he revealed that it was his favorite TV program. When he found out the crew were only using 20 volts, he told them the alligator's skins were too thick for them to feel that and they'd need to use 60 volts. According to Gerry Anderson, the RSPCA employee actually joined the production crew to help them with the alligators, and was using the electrodes on them himself.

-Allegedly, some of the alligators died of pneumonia due to them being left overnight in an unheated water tank. Another one suffered from a broken leg after being shocked.

-At one point Culp says "One move and you're dead!", most likely referencing the title of this episode. This episode and Move - And You're Dead were written by the same person: Alan Pattillo.

-Gordon wears a different diving suit in this episode than what he's previously worn. It's actually the same diving suit Carl had in The Man From MI. 5.

Errors

-When Files is first seen looking out the window, the camera's reflection can be seen in the window.
-When Thunderbird 1 first arrives at the house, there's a shot of it flying over some swamp. It then cuts to a shot of Orchard's group hearing it, then it cuts back to the same shot of Thunderbird 1 flying over the same bit of swamp again.
-Jeff tells Virgil to take pod 6, but later on when the pod door opens it's clearly pod 4.