Dad finishes setting up a bamboo waterline at the base of the bluff when Holly gets frustrated with the unevenness of her clay pots. After Dad recommends using a potter's wheel, his daughter asks him to build one for her. Up by the cave entrance, Will has the kiln all fired up; he calls out impatiently for Holly to hoist up the earthenware. Out in the jungle, the Pakuni gather grapes. Spike is also grazing nearby.

Will asks his sister to toss on the water jug, but Holly claims it would make the basket too heavy. Will then asks for only the dipper, but again his sister fusses that it would get her pots wet. Holly worked a long time on her hand-molded pottery and doesn't want to risk any damage. Just as the basket is half the way up, the normal force of gravity greatly increases. Holly drops the rope, Dad falls from his bench; all of the Marshalls drop to the ground. The anomaly of gravity is wide spread; even Spike and the Pakuni are affected. The brief episode soon subsides, after which the family describes their experiences. Holly felt her pots weigh 100 pounds each, while Will felt as if a giant invisible hand squashed him. The Pakuni begin arguing with Ta threatening to sling grapes at Cha-Ka, but they are pulled to the ground once more during another burst of unusually high gravity. Will drops the rope as he was raising up the basket, and Holly cries out for Dad's help; like the last time however, the force only lasts half of a minute. After Will likens the mysterious force to a carnival ride, Dad realizes that something is increasing their weight. Will notes that the last time strange things happened was when he and Holly messed around with the weather pylons. Dad decides that they had better investigate, so Marshalls head over to find out.

On the way to the pylon, the family runs across Spike, who immediately charges. Luckily, they are spared when another short spell of force hits, since afterward Spike decides it isn't worth it and wanders off. The frightened Pakuni come running up to the Marshalls, seeking an explanation to the phenomenon, while yet another episode occurs, this one even bending over trees. At the pylon, the door is found closed; whoever is inside shut the door behind him. It turns out that no-one is inside, but the control table flashes madly; another burst is accompanied by lightning and skylons. The twirling diamonds fly over the Mist Marsh, home to the Zarn, which clues Dad in on the cause of the problem. Holly sprains her ankle and needs to be brought back to their cave; pressed for time, and needing all of the Pakuni's help, Dad pulls the old "Tom Sawyer" trick. In the Pakuni's native tongue, Dad fibs to Cha-Ka that he and Will used magic to create the strange force, and suggests that Cha-Ka could, too. Ta jealously interrupts that he possesses magic powers, but Dad gives the littlest Paku his "magic" mirror in exchange for helping Holly to the cave. Ta greedily reaches to snatch away the mirror, but Dad insists Ta help Holly too; Ta reluctantly agrees in order to wear the "magic" item. Holly and the Pakuni head back to High Bluff; Will and Dad press on to visit Zarn.

The Marshall men search through the foggy plain of dead trees, and after a very short gravity burst, Dad spots the lights of Zarn's craft. They approach the alien spaceship (a frame of strung Christmas lights); the sparkling Zarn lurks on its deck. Zarn mocks their requests to talk with him by answering Will in Holly's and Dad's voices, explaining to baffled Will that he is reading their minds. (During the mockery, "Holly" mentions "back home in Indiana", though they previously claimed to Confederate soldier Collie that they were from California.) Curiously however, Zarn turns red with pain when he reads their emotions. Dad accuses the Zarn of causing them problems, prompting the glittering being to demonstrate his ship's gravity drive. Zarn notes that his ship will be fully operational for takeoff after making minor adjustments and correcting the lightning-decalibrated instruments. Dad insists that one can't leave the Land of the Lost except through a time doorway, but Zarn adamantly believes otherwise; he has measured the Land's total energy, which his calculations predict can be overcome. Dad argues that doing so would annihilate the Land, and begins climbing aboard after the Zarn remains defiant. When Will and his father walk into the Christmas-tree-light maze, Zarn declares that he has no more time for them, that it makes him "feel bad", so he sicks his robot dinosaur "Fred" on them.

Fred, a 10-foot tall, armless skeletal beast, begins clopping down the hallway, eyes glowing and voice squealing. Will and Dad flee into the Mist Marsh, and hide behind a boulder; for a moment it appears that they have lost him. Will wonders why Zarn hasn't fired up the gravity drive; Dad believes that like them, Fred would fall under its own weight. Fred peeks his head over menacingly, squawking and flashing his eyes; as they run away, Dad gets an idea, and tells his son to head for high ground. The robot beast turn his head very slowly, giving ample time for our heroes to escape. As Dad and Will begin climbing a tall, rocky butte, Will hopes that the pursuant robot will fall and break his neck. They make it to the top, but Fred's squawk grows louder; just then Zarn's gravity drive kicks in. Fred's head pops above the butte top, and he soon towers squealing above them; pinned down by high gravity, Will cries out to his father, but they are spared when a bolt of lighting strikes poor Fred dead. Dad explains to his son why they climbed to high ground; the metal monster would inevitably act as a lightning rod. Will is impressed, but is quick to bring up that his father also claimed that Fred would yield to the gravity drive's force.

As the men head back to Zarn's spacecraft, Dad reminds his son about the being's main weakness. Anger and excitement turn him red with pain; they must concentrate on that to add leverage to their argument. Zarn reads their minds as they approach and commands them to leave; Dad firmly restates the consequences of Zarn's plan. Will brews anger thinking about the Sleestak, which turns the space creature a glittering red. Zarn announces that his ship is ready to put all its power on the line; if necessary, he will break the Land in half to fly free. Dad has Will recall the plight of old Mr. Gerler, a candy store owner robbed by two punks. Will grits his teeth and fumes in a dramatic display; it interferes with Zarn's concentration, causing him to commit errors. The circuits overload, flinging Zarn backward; the gravity drive runs full blast. Zarn tries controlling the malfunctioning drive, but trees bend, cliffsides crumble, and soon his machinery burns out.

As they get back on their feet, Will is amazed that he is still alive. Dad assures that they have survived, but also shows concern for their foe. Zarn answer's Rick Marshall's calls from the wreckage of his ship, and bends over in pain from Dad's apology. Zarn informs Rick Marshall that pity harms him the most, and inhospitably demands that they leave immediately. The space creature refuses Will's offer of friendship, so Dad and his son head back to High Bluff. Stopping for a moment to view the crunched starship, Will sadly guesses that they won the battle. Dad, however, believes that the Land of the Lost won, and that they probably just saved Zarn's life. The Land of the Lost would have self-destructed first, before ever letting the Zarn go; Dad feels that it couldn't have done anything else.