Strolling into a foggy grove of fruit trees, the Marshalls search for food. Acting on Dad's warning about the dangers of poisonous vegetation, Will teases Holly (suggesting they use her as a Guinea pig), which starts a fight. Dad decides to check out the far end of the arbor, leaving the kids to harvest the fruit and continue arguing. Their quarrel, however, is cut short when Will spots a cave entrance nearby. Will decides to investigate, threatening to leave Holly behind if she doesn't join him. Although the kids approach the entrance cautiously, they are stalked by Sleestak hidden in the misty jungle.

Will and Holly continue to bicker while wandering through the tunnels, and finally come to a room with a glowing elongated jewel suspended in a metal tetrahedral frame. Suddenly, the siblings are able to project forces at one another, shoving each other from a distance as their confrontation escalates. Dad shows up, demanding that they stop their fighting; meanwhile, the jewel's intensity increases with the children's hostility. Will and Holly's argument is interrupted only by some approaching Sleestak, sending the Marshalls on the run through the tunnels. They meet up with more and are forced to retreat. The Sleestak corner Dad, Will and Holly back in the room with the suspended jewel, so Dad makes a desperate attempt to keep them at a distance by holding the device out between his family and the monsters. The attempt actually works, prompting Dad to hypothesize that the device and its powers are the key to their way back home. Dad's statement is suddenly corrected by a short, yellow, Sleestak-like being, who describes the device as a divining rod; then, through the suggestive powers of the device, he gestures for the Sleestak to leave.

The creature introduces himself as Enik, the Altrusian. He explains that, like that Marshalls, he has fallen through a dimensional doorway, but unlike the family, he was transported only through time, not space. He has deduced that the Sleestak are his ancestral race; they are all in a time perhaps 100,000 years before is birth. Enik also identifies the device as a Mageti, a tool for travelling through the universe. He affirms Will's conjecture that the Mageti could possibly be used to return the family to Earth, temporarily exciting the Marshalls, but the encounter turns hostile when the Altrusian demands the Mageti for his own use.

Enik insists that they surrender the device, threatening to use force if necessary, although he notes that it might be destroyed in the process. Dad begins asking questions, which that Altrusian somehow answers well before he has a chance to finish speaking. Enik further reveals the purpose of the mageti as a device to locate and repair time doorways. After more of Dad's questions are cut even shorter, Dad and Enik exchange affirmations to questions never voiced; Holly is annoyed and demands to know what's going on. Her father explains that Enik is a telepath, although for courtesy, he agrees to speak aloud. Will asks about the use of force harming the Mageti, so Enik elaborates on the tool's ability to telepathically amplify thought impulses. He also describes a self-destruct mechanism that is triggered when a strong, hostile signal is maintained, to prevent misuse should it fall into the possession of a warring species. Will scoffs at the self-destruct claim, accusing him of trying to trick the Marshalls into surrendering the Mageti quietly. Enik tries to explain that he can't use it to return the family to Earth without knowing the proper coordinates, but Will refuses to believe him. They battle, using the mageti's telekinetic powers, until Will's peaking aggression burns it jewel out. Dad angrily scolds his son, who is nearly in tears and apologizes; the Altrusian comments on the human race's inferiority of failing to control their anger. When Enik mentions that his pendant could function as a Mageti if provided with an external power source to replace the burnt-out crystal, the Marshalls inform him about such jewels being all over the Lost City, and offer to take him there.

While heading through the jungle, they come under the fire of Sleestak missiles. Enik heads over toward their attackers to tell them to leave, not realizing that he is powerless over them without the mageti's influence. Dad pulls Enik down out of the line of fire for the moment, and they flee back toward the children, but the Sleestak are right behind them. When Dad spots Big Alice nearby finishing a meal, he tries to disturb her enough to make her charge in hopes of scaring their opponents off. While he attempts to do so, the Sleestak continue shooting at him; their abysmal marksmanship results in an arrow hitting Alice, who then becomes angry and chases the Sleestak away.

Once Enik and the Marshalls they arrive at the Lost City plaza, the Altrusian realizes the shocking truth: the Sleestak are not his ancestors, but devolved descendants of his own people who failed to control their hate and anger. The Lost City is the ruined remains of his once great civilization.

Dad leads everyone to a chamber fitting Enik's description which houses a crystal-matrix table. After he thanks the Marshalls for their assistance and requests solitude to compute the proper arrangement of crystals in the matrix, Will again becomes angry, having gotten no immediate offer of help in return. Enik attempts to justify his request by stating that there is only one doorway, and that he needs it to return to his own time to warn is race of its impending doom. Upon repeating his request for solitude, Dad reluctantly accepts, but as the Altrusian holds out the pendant to recharge it from the table, Will selfishly snatches it from his hands, demanding that the family be taught how to use it. Enik explains that searching for the coordinates to Earth would exhaust the power of the light jewels, which Will ignores; as Will is about to escape, Enik is forced to summon a fear-invoking fog. One by one, the Marshalls' minds are probed; their deepest fears appear in the mist before them. Enik demands the return of his pendant before dispersing the mist, but Dad manages to partially free his mind from its grip. He warns the Altrusian that only the presence of compassion, not the absence of hatred, can save Enik's people. The Altrusian reflects on this and apologizes for his coldness. Will, knowing that Dad's message applies to everyone, returns the pendant.

Just as the two parties come to a truce, several Sleestak arrive. Enik instructs Will and Holly to get one blue and one green stone from the wall, and hold them together to create a force field. The Altrusian escapes, stating that the Sleestak will not bother him in the Lost City. Though the Marshalls lack confidence, the Sleestak indeed get shocked and frightened off by the field. Dad, looking toward the glowing mist of the time doorway, bids Enik good luck as the family heads back to High Bluff.