In an uncommonly scary 3rd season Sleestak scene, the Sleestak Leader queries a Library Skull for a way to rid their Land of the Marshalls. The Wise One reveals the existence of a special pylon which controls the sun; by using this pylon, all life on the surface of the valley can be extinguished.

Meanwhile, the Marshalls enjoy a day of relaxation -- Uncle Jack sits back lazily sharpening his knife, Cha-Ka feasts on a melon, while Will and Holly play a game of chess on a home-made board. Holly boasts that she's about to beat her brother if they can actually finish a game uninterrupted; Cha-ka likewise boasts that his seed-spitting prowess will soon enable him to spit one from a distance into Spike's eye, after which Spike coincidentally growls in the jungle nearby. The Sleestak have climbed up a bamboo ladder through a perfectly round hole in the floor of the Sun Pylon. The Sleestak soldiers gather around the Leader as he plucks a red crystal from the table; its removal shuts down the entire crystal matrix. The Leader waves a blue crystal in his other claw over the hole in the floor, sealing it shut. Suddenly, the sun begins to flare wildly, prompting Uncle Jack to round up everyone into the Temple. He is unable to explain why the solar flares are occurring, but notes that they should stay inside for their duration; if they do not pass soon, they could seriously threaten all life in the valley. His warning is exemplified as we view the picked-clean bones of a dead fish from an instantly dried-up pond.

The Sleestak Leader again consults the Wise One, getting assured that the Sleestak are safe within their tunnels from the solar flares. When asked if the Sleestak will regain their status as rulers of the valley, the Wise One answers in riddle, "he who would master the valley, need not be its master". The Sleestak Leader then bows respectfully to the Skull before exiting the Library.

After the course of only a few hours at most, the interior of the Temple has risen dramatically in temperature. Uncle Jack attributes the phenomenon to the walls acting like an oven. (Considering the thickness of the stone walls, this should take weeks at the very least; it would never happen at all if the Temple were set into a cliff). Holly pities Cha-Ka for being encumbered by his heavy coat of fur, but when she goes to get him some water she finds that their supply in the Temple has run out. Since the flares do not appear to be letting up soon, Uncle Jack volunteers to go out and fill their animal-bladder water jugs. Will offers to help but his uncle feels it would be pointless for them both to risk the exposure.

Uncle Jack fills the jugs at a low-water-level, algae-choked pond while the flares provide a flashing red glow throughout the jungle. On his way back to the Temple, Jack stumbles upon a pudgy, balding, middle-aged man sitting upon a log, donning an ugly checkered sports jacket, bow tie, orange-tinted sunglasses and a briefcase. In a cheery British accent, the man announces that he had been waiting for Jack, and holding out a business card, introduces himself as William Blandings. He dodges Uncle Jack's question of where he came from, instead offering to help carry the water jugs and politely prodding Jack to hurry. Corny clarinet music reminiscent of a moseying, broken-down horse plays in the background as the two men stroll along the path. They pass near the Lost City plaza, where Grumpy has appeared to battle with Big Alice over scarce water (even though a huge piece of the swamp lies on his side of the crevasse). Blandings feels a duty to remedy the carnivorous dinosaurs' situation, so he pulls out a large tree-branch divining rod from his small briefcase and with more corny music instantly discovers a newly born spring. Big Alice and Grumpy just as instantly become aware of the spring, which Blandings attributes to their keen smell (no way!); he feels content that the beasts fighting has ended, but again dodges baffled Uncle Jack's demand for an explanation to the water-witching.

Back in the Temple, Blandings explains that he is only passing through; he arrived at noon, and must leave promptly at six. He goes on to inform them that during his brief visit, he will assist the Marshalls in any way they need. Uncle Jack points out the hard reality of the their current situation, and complains that they don't know anything about their guest. After Mr. Blandings further dodges questions about his origins, he receives suspicion from Cha-Ka that he is on the side of the Sleestak and suspicion from Holly that he caused the solar flares. Further pressure from Uncle Jack only gets Blandings to reveal that he will always be truthful but can only answer questions as fully as he is permitted. After glossing over Holly's request to learn who he works for, Blandings offers the Marshalls the refreshment of their choice. Holly voices her desire to hypothetically have a chocolate malt, which Blandings miraculously produces for her and Cha-Ka both. Cha-Ka decides he likes it even before tasting it, and even knows how to suck on the straw without being told. When Holly voices in delight that the appearance of the malts is "unbelievable", hers promptly dematerializes from right out of her hand; when she announces that she's changed her belief, it reappears for her to consume. Blandings promises, if time permits, to make them all a gourmet meal, but first must "follow protocol" and take care of some business with the Sleestak. Jack and Holly warn him about the Sleestaks' hostile nature, but Blandings remains unconcerned; Jack then insists on accompanying their guest, which Blandings delightfully welcomes. Holly warns about the solar flares, prompting Blandings to pull out his umbrella. Just then Will returns with supplies; Uncle Jack informs his nephew that he must go with their guest to visit the Sleestak. Will wants to go along, but Jack recommends he stay behind, again due to the undue risks of exposure to the flares.

As the two men mosey through the sweltering jungle heat to more corny clarinet music, Uncle Jack is perplexed that the umbrella somehow provides air conditioning. They arrive at the Lost City Plaza (from the wrong direction if they were coming from the Temple), where Big Alice gives them trouble. After looking around, she charges, but Blandings stops her with a magnetic force-field generating amulet. He assures Uncle Jack that it will work as effectively on the Sleestak, which Blandings proceeds to use once they enter the Lost City's tunnels. Immediately past the entrance, they are in the Cave of the Pit (in direct contradiction to the 1st season episodes); when the Sleestak approach them in the classic slow-staggering, hands-held-out monster style, the force field deflects them, even hurling one Sleestak into the pit! The unfortunate Sleestak hits bottom with a thud, whereupon Jack and Blandings make a joke of it. The remaining Sleestak follow the men from a safe distance into the Library of Skulls, where Blandings opportunistically discovers the Sun Crystal lying unguarded on a pedestal. He rests his amulet upon the pedestal momentarily to examine the crystal, but in doing so the tip of his umbrella, which is tucked under his arm, pushes the amulet off the edge. The glass object shatters on impact with the perfectly flat, clean floor (a cheesy consequence of a dwindling budget), worrying Uncle Jack but not so much Mr. Blandings. The men head out of the Lost City nonchalantly, hoping that the Sleestak will not notice the force field's absence, but when passing through the pit chamber, they run straight into a Sleestak, clueing the Leader in on their vulnerability. The Leader commands his soldiers to attack, whereupon they swarm around Mr. Blandings. Uncle Jack tries to protect Blandings, even jumping on a Sleestak, but the green giants are oblivious to his relatively weak efforts. Blandings insists that Jack save his own life and run, so he does after promising to return. The Sleestak drag the pudgy old Brit over to the pit to throw him in.

Down in the pit, Mr. Blandings casually checks his pocketwatch, which reads 4:45pm. Back at the Temple, Jack shows Will the Crystal, noting its ability to control the sun; unfortunately, he has no idea how to use it. Pressed for time, Jack comes up with an ingenious idea; fill their animal-bladder water bags with the solar-flares-induced high methane concentration swamp gas to create fire bombs to throw at the Sleestak! While Blandings worries about explaining his failure to his supervisors (followed by the corny comment "oh, be quiet, you just had a Sleestak less than an hour ago" in response to the Sleestak god's roar), Jack finishes filling the inverted water bags with swamp gas piped through a hollow reed stuck into the muck. Jack finishes tying off the last animal bladder when Will returns with a thin vine for a wick. They gather up the sacks to head over to the Lost City, where they stop just outside to light a torch with their last remaining matches. After entering the first chamber past the jungle entrance, Will tosses a fireball bomb at a band of Sleestak who stagger toward them at a snail's pace. Since the bomb has made their arrival no secret, Jack calls out to Blandings, who is waiting patiently down in the pit. Mr. Blandings quickly announces his location, whereupon Will and Jack rush into the adjacent pit chamber (actually, the exact same set as the previous room sans one boulder) and toss a rope down to rescue their visiting friend. Will has trouble pulling the portly man up, so Uncle Jack hands Will the torch so he can get Blandings the rest of the way out. More Sleestak show up, only to be fended off by another one of Will's fireball bombs, and in no time Uncle Jack has gotten Mr. Blandings out of the pit. Their guest is very grateful for the Marshalls' rescuing him, but insists he must go deeper into the tunnels to fulfill his mission. When Uncle Jack pressures Mr. Blandings to explain, he admits that he is a "repairman", and today's assignment is the sun. Luckily, Will brought along the Sun Crystal, which Blandings must put back to end the solar flares; the Repairman takes the torch and leads the way, followed by more of that dorky clarinet moseying music.

They reach what appears to be a dead-end passage with a bamboo ladder leading up to a nowhere. While Blandings fumbles in his pockets for the key, Will uses their last fire bomb to hold off more Sleestak for a moment. The relentless Sleestak stagger onward, literally swimming through the air, but the Repairman fortunately produces the key just in time. Uncle Jack fends off the Sleestak with the torch while Will and Blandings scramble up the bamboo ladder. Once Jack is up also, the Repairman seals the hole permanently shut, and assures the Marshalls that the Sleestak can never again enter the Sun Pylon. At Blandings request, Jack glances at his watch to report the time as three minutes to six; Will then hands over the Sun Crystal, whereupon the Repairman replaces it into the matrix. The crystals of the matrix come alive with a glow and hum, signalling that the Repairman's mission has been accomplished just in the nick of time. Blandings uses the key to open the pylon from inside; they all step outside to find not only the solar flares gone, but the infernal heat has instantly dissipated (no way!). Blandings again asks for Jack to check the time; Will takes drink of water, Jack reads 6:00 sharp, and the next thing they know, the Repairman is gone. Uncle Jack is baffled by Mr. Blandings sudden disappearance, but Will points out the Repairman's footprints that lead nowhere. (The pylon has no pyramidal key over the door, and it is never shown whether they close it or not.)

Safe and sound in the Temple, Holly asks Uncle Jack about the mission. She mentions a curious fact that Blandings mentioned during his visit: he said that the Marshalls would get out of the Land of the Lost. Since the Repairman promised to always tell the truth, Will excitedly asks if the man said when. Holly reveals, like most else Blandings mentioned, he wasn't clear enough go give any specific date. The Marshalls giggle sappily as that corny clarinet brings the show to a close.