The Sleestak Leader questions a skull in the Library, seeking the secret to ruling the entire valley. The Leader notes that it is not possible while the Marshalls are around, so the skull reveals the existence of a sacred Orb. The Orb will absorb all light when placed on the valley floor, but it must first be extracted from the pit of the Sleestak god. When asked how to obtain it, the skull replies, "To obtain the Orb, you must master your masters. You must see what cannot be seen. All must be accomplished after dark, before light."

Cha-Ka and Enik accompany Will as he carries unidentified supplies back to the Temple. We join them as Will tries explaining to the Altrusian the human concept of morality. Will likens the idea to sacrifice, where actions are done for the good of others rather than for personal gain; Enik, in a cheap and mistaken infringement by the script writer on Star Trek's Mr. Spock philosophy, deems such behavior "illogical". Just then, the group comes across a pylon along side the path; though Will has travelled this route many times, and Enik many times more, neither of them remember ever seeing a pylon there. Cha-ka thinks the pylon is indeed real, so Will postulates that it had always been invisible. Enik declares the conjecture "illogical", prompting Will to note the innumerable quirks of the Land; in an overuse of more Mr. Spock phrases, the Altrusian labels Will's last comment "logical". Cha-Ka points out that dusk is upon them and that Sleestak are soon to follow, but Will impatiently opens the pylon anyway, promising that he will only be a minute. While Will checks out the pylon, Sleestak quietly capture Cha-Ka and Enik; by the time Will steps out, announcing that it appeared empty, his companions are mysteriously gone. Will assumes that they impatiently left without him, when suddenly he fades to invisibility! After closing the pylon, he picks up his gear tied to a bamboo pole, whistling "Camptown Races" happily as he strolls on home.

Inside the Lost City, Enik stands upon a wooden platform, imprisoned within a net, and suspended above the pit; the Sleestak Leader stands nearby, restraining Cha-Ka from escape. Enik demands that the Sleestak Leader justify his capture; the Leader replies it is part of their plan to obtain the sacred Orb. The Altrusian is quick to bring up that anyone taking the Orb will be devoured; the Leader explains that when the Marshalls hear of their friend's plight, they will make all attempts to rescue him. The Leader orders Cha-Ka to inform the Marshalls, adding that they only have until sunrise; the Paku rushes out of the Sleestak's tunnels as fast as he possibly can. Enik fails to grasp the motive behind the Sleestak's plan, which only results in amusing the Leader.

Back in the Temple, Holly and Uncle Jack make torches. They hear Will's whistling, expecting him to walk in any minute, but they are shocked to see Will's supplies floating hauntingly in the air. Uncle Jack and Holly stare straight ahead, asking Will if it is indeed him; when Will thinks a joke is being played on him, his family explains that to them, he is invisible! Will finds it hard to believe since he can see himself just fine, but then realizes it must have been caused by the new pylon he explored on his way home. They speculate that if Will returns to the pylon, he will become visible again. As Cha-Ka rushes back to the Temple, the Marshalls arrive at the site, but the pylon is nowhere to be found. They worry that until they find the pylon, Will may be stuck invisible indefinitely.

Cha-Ka relays the news of Enik's plight to the Marshall family. Uncle Jack decides that they should save the Altrusian even though he has not been the most reliable of friends. Jack emphasizes their duty to act morally; Will notes that he was just explaining the same idea to their now-imprisoned friend. Will volunteers to save him, but Holly argues that Enik would not act similarly; Uncle Jack wisely enlightens her by stressing that their actions, not Enik's, are their concern. He offers to accompany his nephew, but Will insists on going alone; he can then take full advantage of his invisibility. Jack agrees with Will, but notes that after three hours, they will come to rescue him, too. He hands his nephew an extra knife, which amazes Cha-ka when it disappears under Will's shirt. Holly still wishes that her brother wouldn't go, but Will tells her to consider Enik's position.

The Sleestak Leader announces that time has past, yet Enik's rescuers have not arrived. The Altrusian warns the Leader that he waits for nothing, since he believes that the Marshalls would not endanger their lives to save his own. The Leader boasts that their plan is foolproof, but Enik complains that he still does not understand their motive. The Leader informs his captive that the Orb will give eternal darkness, allowing the Sleestak to rule the valley. Enik tries explaining that the plan cannot possibly work, but the Leader commands silence, proclaiming that the "Voice of Wisdom has spoken".

Will arrives in the Cave of the Pit, where he briefly becomes visible again (this was probably put in the script for Will's invisibility to intermittently fail; it allows viewers of all ages to realize where Will actually is). He clops loudly around the perimeter of the pit to reach the edge nearest his captive Altrusian friend. He whispers to Enik that he is here to rescue him, whereupon the Altrusian predictably calls Will's effort "illogical". When Enik complains that he cannot see his rescuer, Will replies that he is invisible but has no time at present to explain. Will plans to pull the net to the edge of the pit where he can cut his Altrusian friend free, but Enik points out that such a plan is inadequate; first Will must retrieve the sacred Orb from the pit of the Sleestak god. The Altrusian explains the Sleestak's plan to use the Orb to remove all light from the valley; they foolishly believe it will restore their control over the Land. When Will points out the consequential ecological disaster, Enik notes that the Sleestak are too stupid to grasp that. Enik reminds Will that the Sleestak are his future, so he is obligated to protect them, even from themselves. Using Will's invisibility and the Sleestak's stupidity, Enik assures Will that everyone can be saved. Enik calls out to the Sleestak guards, demanding that he speak with the Leader.

It has been three hours since Will left the Temple, so Uncle Jack decides to go after him. Holly and Cha-Ka insists on going, too; reflecting on their feelings, Uncle Jack agrees. They each grab an armful of torches to take with them, but when Jack begins sliding open the huge stone Temple door, several Sleestak just outside claw at them through the crack of the doorway. Uncle Jack quickly closes the door, so they are safe for the moment, but Cha-Ka and Holly worry how they can go rescue Will. With no telling how many Sleestak await in the jungle outside the Temple, Uncle Jack is at a loss to plan their next move to rescue their family member and Altrusian friend.

The Sleestak Leader is angry to have been summoned back to the Cave of the Pit. Enik fibs that, like he had prophecised, the Marshalls did not come to his rescue; the Leader notes that it does not matter, since then their captive must retrieve the Orb from the pit. Enik brings up that there exists an alternative; within his chambers lies the Mystic Altrusian Rope, which will obey Enik's every bidding. The Leader is skeptical, never having heard of such an item, but upon Enik's request, sends a Sleestak to retrieve it.

Back in the Temple, Uncle Jack suddenly gets an idea -- digging in the survival kit, he pulls out a flare gun! After they open the door just a hair, he sticks the gun out just enough to fire it. It lights up the area with an intense, flickering glow, giving them the chance to run past the temporarily blinded Sleestak. (The Marshalls strangely don't seem to mind leaving the Temple door wide open.) Grumpy is upset as well by the flare, but its light quickly burns out.

The Sleestak Leader holds out a rope retrieved from Enik's chambers, whereupon the Altrusian confirms (falsely) that it is the Mystic Rope. He instructs the Leader to anchor one end on a rock along the pit's ledge; the other is to be lowered down into the fog. Enik additionally instructs Will to climb down the rope and retrieve the sacred Orb. The Leader questions why Enik is murmuring, so the Altrusian makes the excuse that he was reciting a prayer to help work the rope's magic. When the ropes pulls taut under Will's weight, Enik points it out as evidence of the rope's search. Down in the pit, Will (at least his knife) hunts around in the fog for the Orb. Meanwhile, Uncle Jack, Holly and Cha-ka are attacked by Sleestak in the jungle, prompting Jack to fire off another shot from his flare gun. They rush on to the Lost City while the Sleestak cover their eyes and stumble to the ground.

The Sleestak Leader grows impatient and yanks on the Mystic Rope. Enik warns against doing so, insisting it remain undisturbed to work its magic. The Leader rebuts with the reminder that, for Enik's sake, it must produce the Orb soon. Will rummages around at the bottom of the fog-filled pit, knocking down balanced boulders to reach another chamber. Again he reappears briefly as the invisibility starts to wear off. Jack, Holly and Cha-ka reach the plaza of the Lost City, but Big Alice guards it well. Uncle Jack proposes that they go around through the jungle in an attempt to get by the Allosaur; Cha-Ka adds that sunrise is near. Back in the pit, Will has found the sacred Orb of the Sleestak: it is a soft-ball sized, red sparkly ball, which looks rather like a Christmas ornament. He carries it over to the rope to start the climb up. The Leader is displeased that the god roars so angrily; he attributes the god's disturbance to the Mystic Altrusian Rope. He holds Enik accountable for offending their god, and though it is not yet sunrise, sentences the Altrusian to immediate sacrifice. Fortunately, the rope pulls taut once more, since Will (whose invisibility is now fading more frequently) begins his climb out the pit. The Sleestak Leader grants Enik some time to see if his rope has really found the sacred Orb. The Sleestak together pull the rope out of the pit; the Leader is puzzled by the Orb's unusually heavy weight. The Orb floats up and out of the fog, but floats away, out of reach of the Sleestak Leader's clutches. Enik informs him that the Orb will not be theirs until he is set free, so the Leader commands his soldiers to release their captive. The Orb then floats into the Sleestak Leader's claws, just as the Sleestak help the Altrusian out of the net ("that was most uncomfortable", gripes Enik).

Just after the Sleestak Leader, gazing at his newly acquired Orb, announces his race's restoration as rulers of the valley, Will's invisibility intermittently fails, prompting Enik to foolishly announce that fact as well. Will is shocked to realize that Enik endangered the Marshall family, but the Altrusian claims (illogically) that seeing to his own personal interests was "logical". Will disappears once more, so he snatches the Orb from the Leader's hands; he drops it into the pit, where it shatters upon hitting bottom. Will's invisibility wears off completely, prompting the Leader to order his sacrifice. Will struggles with the Sleestak, but is aided by Uncle Jack who has just arrived. Enik, however, orders everyone to stop the conflict, since the sun has risen, and with it the chance to fulfill the prophecy. The Orb has been destroyed, having shattered in the pit; any further struggle will only mean loss to both sides. The Altrusian goes on to note that there will be another time, another prophecy. Will angrily demands an explanation for why, at first Enik endangered the Marshall's lives, then lobby for their safety; the Altrusian replies that both actions were completely logical when considering his own personal interests. Jack calls upon his family to leave, grumbling that Enik will never comprehend the meaning of morality; Enik aggressively refutes the statement, claiming that during Will's rescue attempt that he did understand morality, but that he considers it "illogical". With the Sleestak complying with Enik's request, the Marshalls are allowed to return to their Temple.

While the Marshalls eat breakfast, Uncle Jack admits that he has no clue regarding the pylon's or Will's invisibility, though Will proposes it may have been connected with the Sleestak prophecy. Jack adds that regardless, the cause, like many other bizarre phenomenon of the Land, is most likely beyond their comprehension. Somewhat embarrassed and teased by Cha-Ka, Holly asks her brother to reveal what the god of the pit looked like. (Why would Holly ask this question? She was the only one to see the monster; she attacked it while rescuing Will [Elsewhen]). Will confesses that he never saw it, but since Holly is so interested and gullible, he makes up a the story that he touched the beast, to find it was cold and incredibly slimy. Uncle Jack begins to laugh, finally clueing Holly in on the prank.