Holly tries teaching Cha-Ka to play "Rock-a-bye Baby" on a bamboo recorder, but the hairy little simpleton can do no more than obnoxiously blow a single, unstopped note through the instrument. Holly gripes to Dad that Cha-Ka is either dumb or just plain stubborn, but her father explains how man evolved over millions of years just to count or beat on a drum. He believes that "it is not yet time for the Pakuni to learn how to make music"; Cha-Ka hams it up by a making goofy face that shows his crooked teeth. Dad mentions that he talked with Will about exploring the Temple the kids found near the Lost City, and they prepare to leave when Will comes down from the cave. Will points out that Big Alice is most likely hunting since Junior has hatched, so they should not have to deal with her harassment. The Marshalls have to shout at Cha-Ka to get his attention away from his newly discovered toy; when Dad asks the Paku to join them, he excitedly agrees.

During the trek through the jungle, Cha-Ka continues to irritate everyone by blowing monotonously through the recorder. While Holly stops to tie her shoe, Will notes how Cha-Ka is a far cry from Mozart, who could play classical piano at the young age of 4. When he asks Dad where the gift of genius comes from, Holly can't resist but to answer "from their mothers!" Will yanks the instrument from the Paku's mouth, threatening sweetly to break it into a million pieces if he does not stay quiet. As they continue on, with a smirk on his face, Cha-Ka taunts Will by increasing the fervor of his playing.

The Marshalls and Cha-Ka arrive at the Lost City to find that Big Alice is still around. Dad shushes the spaced-out Paku, who is still blowing the bamboo flute non-stop; after hiding behind the archway, they get lucky when Alice and Junior leave. Will loses his temper, snatching the recorder from Cha-Ka's hands and throwing it into the jungle.

As the group walks over to the pillars, Will asks Dad if the Sleestak could have built the Temple. Dad thinks that the scale is correct, but something indescribable makes him believe otherwise. Walking past a floor-set sundial to the dais against the wall, Will comments on the height of the architecture, and notices what appears to be a sealed door. At first they don't find any type of door handle or latch, but then Dad finds a sculptured relief high on the wall of two human hands touching. One hand is convex, the other concave; Holly curiously asks to be lifted up so she can try to fit her own hand in it. Will teasingly scares her by yelling "watch out!" just before she tries it. Her hand fits perfectly, but she pulls it away quickly; she thinks she felt a tingle, but then writes it off to her imagination. As they look around for other evidence of human builders of the temple, they first examine an obelisk, and then discover a large stone etched with writing. Meanwhile, Cha-Ka leans a log against the wall to reach his hand up to the carved relief; the Marshalls hear his screech, and see that the door has opened up.

Holly rushes to Cha-Ka's aid, while Dad moves away the fallen log; luckily, the little Paku is unhurt. As they walk into the chamber, it automatically illuminates, causing Holly to become scared. Will assures her that from the must and dust, the previous occupants are long gone. They notice a large, smooth-topped table in the room's center; Dad dubs it "the great-grand-daddy of all matrix tables" ("only streamlined!", adds his son). Will blows off the dust and reaches his hand out to toy with it, but Dad quickly stops him, telling everyone to take a look around first. Everyone begins exploring the Temple, while Cha-Ka gazes at the table; suddenly a ruby ring materializes on its surface, where the Paku quickly snatches it up. As soon as Cha-Ka mutters in awe at the ring, Holly grabs it and puts it on. When Dad tells her to remove it and place it back upon the table, try as she might, it won't come off. Will, wandering into a connected chamber, sees someone else across the room; with Holly clinging to her father in fear, Dad comes over to take a look. He realizes it is a statue; the room they are in is an art gallery. Looking at the statue and the other sculptures in the room, Will now feels certain that humans must have built the Temple. Back in the first chamber, Cha-Ka touches the giant matrix table; each section glows and beeps as his hand contacts that region. Outside, we see that Cha-Ka's toying summons a humming light up in the sky. Believing that he can create magic, the Paku rapidly touches all of the table's squares; the table beeps and flashes wildly while the glow in the sky twists and knots. He shouts for the Marshalls to quickly come see what he has found, but the table fails to respond when he attempts to demonstrate it for them. Puzzled and embarrassed, he falsely says it was nothing. Dad notes that it is late, and that they are in Sleestak territory; he promises that they will come back tomorrow, and leads the family back outside. After Cha-Ka trails the group out, the table lights up on its own; a swirling column of small colored balls slowly appears, and within it a tall, red, glowing being materializes. The faceless humanoid beats in a constant, midrange pulse, and begins to wander out to the plaza, frightening Big Alice and Junior away.

Back at the cave, Dad tries pulling the ring off Holly's finger, but it still won't budge. She can't feel thing in her whole hand, worrying Dad that her circulation has been cut off. Suddenly Cha-Ka hears a voice calling out his name, but the Marshalls don't seem to notice. Ta and Sa, walking through the jungle, hear their names called out, too. Ta points out to Sa the swirling streaks up in the sky, but they cower and flee when they see the tall red being approaching.

After Ta and Sa arrive at the Marshall's cave, Will points out to Dad the light and streaks up in the sky. Holly unexpectedly hears the voice calling out her name, but only the Pakuni seemed to have heard it. When asked by Dad to tell him what she heard, Holly suddenly becomes entranced, and in a sleepy voice explains that "the Builder wants his ring". She waves her ring-hand out in front of her, whereupon Will notes that she couldn't move it just a moment before. Holly reveals that the ring must be returned to the Temple, so Dad decides they have no choice but to bring her back there right away. Cha-ka bravely volunteers to help them bring Holly.

As the Marshalls and the Pakuni travel through the jungle to the Temple, Holly's leg begins to weaken and hurt. The Pakuni once again here their names called and become frightened. Holly continues to weaken, but Dad insists that they keep moving. Meanwhile, the red sky-streaks keep swirling all around. Will trips on a taut vine and pulls his knee in the fall; he says he'll follow behind using a big stick as a crutch. Dad, however, warns that the Lost City is dangerous place, and recommends his son get back to their cave. Temporarily tended to by the Pakuni, Holly is becoming dizzy and frightened; Dad helps her along, assuring her that they're almost there. On his way back to the cave, Will encounters the beating red being. Will warns it to stay back, and explains that they are returning the ring, but ominous and unspoken, it continues to march forward. After Will falls in retreat, he screams out to warn his family; the Builder proclaims "No, Will Marshall, you have already proven yourself, it is not your time" and zaps Will unconscious. Dad, Holly and the Pakuni all hear the voice mockingly call out "Cha-Ka, Ta, Sa"; when Dad questions the voice, he gets the same message and fate as Will. Holly falls exhausted to the ground, and passes out as well. Cha-Ka begs Ta and Sa to help him bring Holly the rest of the way, but the spooked Pakuni only shake their heads. The voice utters "you failed" in disappointment to the older two Pakuni, while Cha-Ka adamantly struggles to drag Holly along. The Builder proclaims that Cha-ka has failed, too, but the faithful little Paku continues fruitlessly to try. The Builder sternly repeats his judgement, but the monkey-boy loyally protects Holly from any harm. The voice calls out, asking Cha-Ka for the ring; after he effortlessly removes it from Holly's finger he holds it out on his palm, where it dematerializes in an instant. The tall being steps in front of the Paku, then shrinks down to his size; it suddenly transforms into a yellow-suited, human Cha-Ka who announces "it is your time, Cha-Ka". The Paku holds out his hand, and they both put palms face-to-face. The human boy begins to back up when the ring rematerializes on Cha-Ka's finger; the Builder walks away slowly, looking back at his Paku twin. The humming light up in the sky vanishes as well.

Back in the Marshall's cave, the kids are amazed that their injuries have miraculously disappeared. Will asks Dad what the Builder meant when he declared that it wasn't their time, but his father can only guess that it must have gotten what it wanted. Just as Will wonders who's time it actually is, we see and hear Cha-Ka out in the jungle, perfectly playing "Rock-a-bye Baby" on the recorder.