Late one evening, the Marshalls are fleeing through the jungle from an unspecified menace; they find that they have roamed into an unexplored, forbidden area. Holly stops to rest for a moment, but they are forced to continue on when unseen monkey creatures harass the family by tossing tree branches at them and screeching angrily. When the Marshalls stop again to notice the sudden unnatural silence, Uncle Jack spots a creature standing in a clearing facing a mountain pass. Will discerns in the darkness that the creature looks like a short Sleestak; Holly guesses that the being is waiting for someone. Jack has a hunch that the "being" is inanimate, which he proves by stoning the "creature" and failing to get a reaction. As they approach the object, the family hears a humanoid's roar; though Holly is frightened and wishes to flee, Jack insists that they investigate. Will's examination reveals the "creature" to be a wooden statue of an Altrusian torso which bears rune-like writing on its chest. Holly is puzzled why the Sleestak would place such a statue in this location; Jack hypothesizes that it is a sign post leading the way through the mountain pass. Grumpy's nearby roar reminds the Marshalls that they are not safe; this disappoints Jack, who wanted time do decipher the symbols. Will proposes that they take the statue with them, and in their haste, Jack agrees, so they rush off lugging the object.

Back at the Temple, Cha-Ka is preparing dinner; when Jack and Will set down the statue, the disturbed Paku threatens it with an explosive light crystal. Holly tries to calm Cha-Ka by letting him know it is only a statue, but the spooked Paku still considers it evil. Will points out that Enik may have some knowledge regarding its origin and purpose, so he plans to visit their Altrusian friend early in the morning. Jack advises they get a good night's sleep; he will also try to decipher it himself the next day. Being considerate of Cha-Ka's fears, the Marshall men carry the statue outside. As the men take a short break, Will wonders if the inscription on the statue holds any clue to helping the Marshalls return home. The conversation with his uncle turns to wondering how they would explain their experiences in the Land to everyone should they ever get home. Jack makes the corny comment that they could always write a book or make a movie of their adventures.

Over at the mountain pass, the humanoid creature's roars grow louder until we see the pathetic "monster", a generic costume of a silver-haired bigfoot, appear at the far end of the mountain pass. The Monster cautiously tests the ground in front of him as if it were hot or about to explode; when it proves safe, the Monster roars with a tone of victory and charges ahead with arms in the air as if scaring away little kids.

The creature lurks outside the Marshall's Temple, waking Holly; she in turn wakes up Cha-Ka, who simply tells her to go back to sleep. The hairy monster then heads over to the Lost City, where he terrorizes the Sleestak, chasing them down the tunnels. One brave Sleestak tries to grapple with the beast, but is easily tossed to the side by the creature. The Monster heads into the Sleestak Pit chamber, where he destroys their artifacts and knocks an ornamental obelisk into the pit. The Sleestak Leader awakens Enik, who is sleeping in his chambers (a humble cavern with a stone ledge for a bed), to alert him that "Kona", the dreaded creature of the high country, has come inflicting wanton destruction. The Leader urges the Altrusian to act quickly, before the creature reaches the Sleestak Hatching Chamber; Enik grabs a small device from his belongings and charges off with the Leader to confront Kona. Enik zaps the creature with a 10-second pulse from a "phaser", after which the monster flees. The Sleestak Leader notes that Kona is gone, but is likely to soon return; Enik is shocked that something must have happened to the "Ancient Guardian". The Leader points out that the Guardian has protected them since a time beyond memory; the Altrusian advises that they consult the Old Ones.

In the Library of Skulls, Enik questions the Voice of Wisdom why the Ancient Guardian has apparently forsaken them. The Voice replies, "To guard is not to turn away; to see is not to sleep; those who see least, see best; those who know most, know nothing." The Leader interprets the Voice of Wisdom's message as indicating that the Marshalls have stolen the Guardian, so the Sleestak must retrieve it and eliminate the family. Enik stands up for the Marshalls, believing that they did not know the Guardian's purpose; he will speak to them, since it is in his benefit to do so. (At least the script writer and story editor realized that they had to justify Enik's compassion, though it is never specified how the Altrusian intends to benefit from sparing the Marshsalls.)

During the latter part of the night, Sleestak warriors quietly retrieve the Guardian from the Temple courtyard. When the Marshalls awaken at dawn, they find their statue is gone. Enik arrives to warn Uncle Jack that the Sleestak are angry at his family for taking the Ancient Guardian. In a tone unusually calm for the Altrusian in the 3rd season, Enik explains that the Old Ones made the Ancient Guardian. They placed it upon the pedestal by the mountain pass to keep the hairy monster from the highlands from coming down to their valley. The Altrusian does not know how the Ancient Guardian works, but warns the Marshalls that they must never disturb it again. As Enik marches off, Will and Jack ponder the Altrusian's claim; Uncle Jack believes the Guardian's powers are just an old Sleestak myth. He rounds up the gang to get to their breakfast, since afterward he wants to return to the mountain pass to examine the "high level equations" on the statue. (Uncle Jack really screws up here -- it hasn't been revealed yet in the plot that the runes are mathematical symbols.)

Kona once again roars madly through the Lost City, terrorizing the Sleestak and leaving trail of destruction. With the Sleestak Leader by his side, Enik fires his phaser at the beast, but it does not stop the monster, who simply shoves them aside. Kona rushes into the Egg Cave, where he smashes some eggs, drinks one, and then runs off with an armful. (This has got to be one the series' worst scenes; not only is the monster costume is incredibly fake, but the eggs don't even break when Kona throws them to the ground or when he "cracks one open" to unconvincingly drink from it.)

Back at the Temple, Will and Jack have finished repairing the statue, so Jack decides to take a "scientific approach" to deciphering the runes. He brings out a slate with some chalk to write with, but first tries to assure Cha-ka that the statue will not hurt him. The Paku remains suspicious, hiding at the edge of the courtyard, fearful that the statue could be magic and come to life. While Holly and Will talk about their home on Earth, Uncle Jack examines the Ancient Guardian. He rotates it slowly to get a view of it from all sides; when it is in a specific position, "invisible" beams shoot from its eyes into the jungle, causing the foliage near Cha-Ka to explode in flames. No one noticed how the fire started, but after Jack puts it out, Will accuses Cha-Ka of starting it. The Paku denies the charge, but wanders off feeling shunned. Holly then toys with the writing that Uncle Jack copied on to his chalkboard; she notices that the rearrangement makes the symbols appear as arithmetic. Jack rushes over to take a good look, where he verifies that the new arrangement shows equations of algebra. He claims that he overlooked her interpretation since he was looking for "complex engineering equations". Will asks his Uncle to cut through the jargon and get to the point of what the equations say. Jack recognizes the equations as those from the science of optics, whereupon they all conclude that the statue's eyes started the fire. He isn't sure just how the statue's eyes worked, but with a little study of the equations he should understand the details. (This is all extremely hard to swallow. First of all, Enik is very knowledgeable about the physics of ultra-dimensional matrices [Circle], so he should certainly know something about the mathematics of optics. Second, why would the rearranged runes turn into mathematical symbols used by Western Civilization earthlings? Third, how did Jack know in the first place to attempt to interpret the runes as mathematical symbols? Fourth, the mathematics of (Fresnel) optics involves advanced calculus, so why would these equations appear as simple algebra?)

In the Library of Skulls the Sleestak Leader is in a state of panic, claiming to Enik and the Voice of Wisdom that Kona will return. At first he states that Kona will steal their eggs and destroy them, then decides to ask the Voice of Wisdom of their fate (the Sleestak Leader stumbles through his lines). The Voice predicts that the Sleestak are doomed, but gives the following line of advice: "He who seeks help, seeks destruction; he who offers help, seeks all." The Leader believes that the Guardian's failure to serve them is a defiant expression of its anger over being stolen by the humans. Enik again sticks up for Marshalls, claiming that they did not know the consequences of their actions, but the Leader insists that they must be punished, after which he stomps out of the room.

Jack works at the algebraic optic equations, and in no time feels that he has figured them out. He explains to Will that "a concentration of lenses inside the statue's eyes are angled just the right way" to create a "solar energy machine". "The eyes concentrate the sun's rays through a system of lenses and prisms and then beam it out through them", he claims; he confirms Holly's interpretation of the effect as a "heat ray". Jack decides to call it a day, saying he will go over the equations fully in the morning.

Later that night, when Jack steps out into the Temple courtyard to fuel the braziers, a band of Sleestak sneak up behind him. They throw him aside, then clop into the Temple, but in the ensuing battle, Cha-Ka drives them off with exploding light crystals. After the conflict, the family steps outside to find that Enik has arrived to warn them yet again. Enik explains to the Marshalls that the Kona steals the Sleestak eggs, and that the race is doomed since soon there will be no more Sleestak young. Jack informs Enik that the Sleestak came after his family, and apparently ignored the Ancient Guardian; the Altrusian explains that the Sleestak believe that the Marshall's removal of the Guardian has drained it of its powers. Jack tells Enik that he believes he has discovered the secret of the Guardian's power, and promises to restore the statue in the morning. Enik accepts Uncle Jack's promise, and returns to the Lost City to inform the Sleestak.

The next morning, the family has arrived at the mountain pass; Holly urges Will and Jack to hurry as they replace the statue on the pedestal. Jack reminds his nephew that the Guardian must be positioned exactly right to work; Will points out the grooves on the pedestal top must be there to align the statue. Once in place, the Guardian's eyes begin to glow and hum; a nearly invisible "heat ray" turns the rocks in the mountain pass a glowing red. Just then Kona arrives to find the ground too hot to walk on; he shakes his fist and roars, as if saying, "I'll get you next time!" (One glaring flaw about the operation of the Ancient Guardian: what keeps Kona from coming through on a cloudy day or at night? The guardian could store excess energy to provide continual heat upon the rocks, but Jack never indicated that such a feature existed.)

That evening, the Marshalls reflect on the experience as they relax in the Temple courtyard. Uncle Jack feels that they have learned an important lesson: leave things alone that you don't understand (he may have just learned that lesson, but the kids learned it from Dad in the 1st season after messing with the pylons and explosive light crystals). Will is surprised that the Sleestak must have been much more technologically advanced at one time in the past (why is this surprising? Didn't Enik's people build it?) Will mocks Enik by impersonating the Altrusian lecturing the family on the "logical" reasons for restoring the Guardian. Holly asks her brother to play them a song, so Will feigns playing his junk guitar and sings them a lame tune.