The Marshalls are busy making tools in their cave when Dad asks if anyone will fill the water jugs. Holly volunteers, but Dad tells Will to go with her for her own safety. Holly protests but Dad stands firm. At first Will reluctantly agrees, then offers to carry the water jugs for her, but Holly insists on carrying them herself. Will suggests that they take a shortcut so they race out of the cave.

Will leads Holly across a crevasse she's never seen before, which is bridged by a fallen stone slab. Holly accuses Will of exploring without Dad against his wishes; Will admits to it, but insists it's still a shortcut.

Will and Holly soon stumble upon the ruins of the Lost City, a plaza with an arched entrance, crumbling rectangular pillars, and three cave entrances set into a mountainside. Their awe and speculation about its creators and inhabitants is interrupted by Holly's sighting of a huge Allosaurus, the guardian of the Lost City's plaza. Holly quickly nick-names her "Big Alice". Will convinces Holly, who wants to go inside, that they'd better tell Dad first about what they've found, so they head back across the crevasse.

As Will and Holly head back through the jungle towards the cave, Cha-Ka, still wearing his leg splint from the previous episode, sees them approaching and hides behind a log. Using a mirror, Will sends a sun-signal in Morse code to Dad about the Lost City. They receive a reply to be careful, fetch the water, and meet him at the crevasse. As they head out for the water, Holly first hears something in the jungle (Cha-Ka), then finds fresh Pakuni tracks in a clearing of heavily oxidized sand. Hoping they'll find Cha-Ka, Holly insists on following the tracks. Will, more doubtful, disagrees, but leads in the direction of the tracks anyway. Cha-Ka follows them while they follow his tracks; everyone ends up going in circles. Finally figuring it out, Will and Holly hide behind a rock, surprising Cha-Ka and scaring him away. Will chases after him until they both stumble upon "Spike" (or "amenda" as Cha-Ka calls him) -- a grazing Triceratops. Spike charges after them, passing them by as they dive behind a rock. Will and Holly reestablish their friendship with Cha-Ka, cutting loose the splint from his leg and offering him water, but then abandon him as they head out to meet Dad. They arrive at the crevasse, with Dad nowhere in sight, and decide to head on to the Lost City. Meanwhile, Cha-Ka follows.

Will and Holly find themselves lost in the jungle and wander into a cluster of vine-covered pillars. The surroundings are dimly lit, the ground covered with a blanket of fog. They notice a warning written in chalk on one of the pillars. When Will reads it aloud -- Beware of Sleestak -- Cha-Ka becomes alarmed, repeatedly exclaiming "sarisataka". Spooked, Cha-Ka runs off they way they came, but Will and Holly head on, with three Sleestak sneaking up behind them. Will and Holly find themselves walking out onto the Lost City plaza from the left side near the cave entrances, and are quickly captured by the band of 7-foot tall, gargantuan-footed Sleestak.

Cha-Ka runs stumbling through the jungle, encountering Dad at the crevasse. Speaking in Pakuni, he tries to tell Dad that the Sleestak have captured Will and Holly; Dad doesn't understand everything yet realizes that Will and Holly are in some sort of danger. Cha-Ka leads Dad toward the Lost City.

Will and Holly have been imprisoned within the Lost City by the Sleestak; they are sitting on a wooden platform encased in a net, suspended by a rope over a fog filled pit. Holly confesses that she is truly frightened. As a Sleestak enters the pit chamber, Will guesses that their captors are indeed the Sleestak mentioned in the warning on the jungle pillar. A deep roar bellows from below the fog, leading Will and Holly to fear that the Sleestak plan to sacrifice them as a meal for their "god".

Meanwhile, when Cha-Ka and Dad arrive at the jungle pillars, Cha-Ka points out the Beware of Sleestak warning. Cha-Ka attempts to convey a Sleestak's height by stretching his arms out vertically. They then head on toward the plaza.

Back at the sleestak pit, Will postulates that they could escape by ripping the net apart. Holly reminds him about the Sleestak God below, yet Will demonstrates that they can rock the platform over to the edge of the pit and could jump off. Holly refuses at first, but after another roar bellows from the pit she quickly changes her mind. Will manages to tear open the net, rock the platform over to the ledge, jump out, and help out a frightened Holly as well, but their escape is foiled by the sudden appearance of their Sleestak captors.

Cha-Ka leads Dad onto the Lost City plaza and finds the empty water jugs dropped when the kids were captured. Big Alice shows up, sending them for cover between the blocks of a fallen pillar. Exclaiming "ota" (fire), Cha-Ka holds up a stick to be used as a torch. They just make it into the central cave entrance, barely escaping from Big Alice.

The Sleestak have once again imprisoned Will and Holly, this time tying a large rope around the net to prevent another escape. When the Sleestak God roars, Will wonders what the monster is, commenting on its awful smell. Will praises Holly for her bravery through the ordeal.

Dad and Cha-Ka head into the tunnels of the Lost City. Although Dad is carrying a torch for light and protection, Cha-Ka is still jittery and frightened. Just then, a Sleestak sneaks up behind them, but Dad is able to fend it off with the torch. Heading on and calling out for his children, Dad stumbles upon another wandering Sleestak, again scaring it off with the flame.

Once Will sees the glow of Dad's torch, the kids quickly establish voice contact with their father. Will and Holly scream in terror for their father's safety as Dad and Cha-Ka manage to enter the pit chamber by fighting off another Sleestak. Cha-Ka then holds the torch, backing a cringing Sleestak into a corner, giving Dad a chance to hand his knife to Will. While Will cuts open the net, a second Sleestak stalks Dad, and a third enters the room heading toward Cha-Ka. Cha-Ka tries to fend both of them off, but one of the Sleestak knocks the torch out of Cha-Ka's hands. Dad reaches for the torch, but in the scuffle with the Sleestak it gets pushed into the pit, invoking a blinding light and deafening roar from below. With the Sleestak frightened off by the light, Dad quickly helps Will and Holly out of the net. Pulling Cha-Ka away from the lighter he's playing with, they run out of the Lost City as fast as they can.

The episode closes as Cha-Ka and the Marshalls stroll across the crevasse and through the jungle. Cha-Ka spots Pakuni prints in the sand and bids them goodbye. Dad holds up the empty water jugs, reminding Will and Holly that they still need to complete their chore.