Doc has decided to make a deal with a contractor that would make him and Sprocket rich. The deal is that they use the limestone caverns below Doc's house as storage for industrial waste. Sprocket protests, but Doc says he'll call the deal off if Sprocket can prove that there are creatures living in those caverns. Until then, Doc has authorized a test injection of 100 gallons of the waste ...
Meanwhile, it's a really hot day, and the pool in the Great Hall has gone dry, so the Fraggles go down to Waterwheel Cavern to get the water, pail by pail, to refill it. All except for Boober, who would rather be by himself. Boober smells something wrong in the water, and by the time he gets to the pool to warn the others that the water's been poisoned, they're all splashing around in the pool. Later on, when Boober takes his friends to the place where he smelled the awful smell, he observes that they're all getting very sick, and now they're all counting on him to save the day. He tries to seek help from the Trash Heap, who clues him in to who's poisoning the water (he previously thought it was the Gorgs).
Incidentally, the Gorgs are having similar problems: Ma and Pa have gone to the creek and quickly come back with itching feet, and the water that Junior has sprinkled on the radishes has made them turn brown and wilt. Pa grabs ahold of Boober because he thinks the Fraggles are the cause of this, but Boober tells him it's the Silly Creatures' fault. Pa then forces Boober to go to Outer Space and save the day, which he does. His initial reaction is to panic and hide, but thanks to the Trash Heap, he knows that's not an option, especially since the Fraggles are getting sicker and sicker. As Pa puts it, "I'll make it country simple, you little fur ball: if you don't get the message across to these jokers, you might as well kiss your hat goodbye. Because you know what? Without clean water, we are all going to die!"
Boober arrives at the entrance to Outer Space while Doc and Sprocket are taking a walk. Not knowing the room is empty, he begs for them to stop polluting the water, knowing that Silly Creatures respond well to groveling. Finally, he gets an idea to prove that Fraggles exist: he gathers all of Uncle Matt's postcards, ties them in a bundle, and leaves them at the hole. Sprocket finds these cards, and gives them to Doc, who becomes puzzled. Later, Doc tells Sprocket he has declined on the big-money offer that the contractor made to him, and that he also told the man to inject 100 gallons of spring water into the caverns, thus creating a happy ending for all.
When this episode was shown on HBO it had a different end credit sequence, which is rarely seen nowadays (it was removed from Odyssey as well
).