Holly awakens one morning to find Grumpy chomping on their cave curtain! Her scream awakens Dad and Will, but before they can give Grumpy the Flyswatter, he wanders off. As they attempt to rehang the curtain, Will notices that Grumpy actually ate some of it! When Holly explains that the curtain contains "dinosaur nip", Dad realizes that the patches of it near High Bluff must be what's drawing Grumpy to their cave so often. Holly innocently suggests that they weed it all out and throw it in the crevasse, much to Will's displeasure, and her following attempts to discredit the proposal fail; their father drafts them to start on the chore promptly after breakfast.

While the kids weed out a patch of the nip, Will grumbles about missing out on a day of fishing. Dad drops by to check on them, warning the two about Grumpy and asking Will if they can handle this patch. After his son assures Dad that they can do it alone, their father treks out to tackle the one south of the cave. When Holly struggles to lift up her tied bundles of nip, Will warns her that she can't carry as much as he can; his sister, however, insists that girls can do anything boys can. Lugging the bamboo poles (to which the bundles are tied) over their shoulders like a yoke, they trod over to the crevasse, but not without inadvertently attracting Grumpy with the concentrated scent of the weed.

The children cautiously pitch the bundled nip into the chasm and decide to take a rest by some rocks along the cliff behind them after Holly dramatically feigns exhaustion. During their break, Will discovers a human body buried in the rubble; it turns out to be a dinosaur-nip stuffed dummy, clothed in an 18th century coat and knickers. They detect something in the dummy's coat pocket, but are interrupted when both Grumpy and Big Alice show up, having been attracted by the fragrant ferns. The kids take cover behind the rocks, and the two mighty beasts begin roaring at one another from each side of the crevasse, each defending its own territory. Dad hears the battle from the south patch and rushes off to investigate. Will instructs Holly to escape immediately, while the two great lizards are still busy with each other; he will follow with the dummy as soon as he gets the chance. Holly meets up with Dad, stating that her brother will be along soon with a dummy, and Will promptly arrives, displaying the dummy and mentioning a book in the coat pocket. The Marshalls head back to High Bluff where they can read it in safety.

Once in their cave, Dad begins reading from the book, which appears to be some sort of diary. The journal mentions the Pakuni, as well as the Sleestak; the latter the author had actually named after Major Joshua Sleestak, and was evidently responsible for writing the warning on the jungle pillar. The book goes on to mention a missing companion, Harry Potts. The author created the dummy to put beside a fire in order to mislead the carnivorous dinosaurs, creating a diversion sufficient enough to get to "the cave where the pillars end". The diary concludes by promising further clues on rocks and paper. Will excitedly urges they search for the continuation of the journal in the Lost City, assuring Holly that they will be safe from the Sleestak since they are still in hibernation. Dad agrees, but warns that the Sleestak's dormant season is almost over (how does he know?). They take the aromatic dummy with them to drop off along the way, and begin their adventurous quest.

As the Marshalls cross the crevasse, they are followed by their old nemesis Grumpy. To their surprise, the beast continues to follow, even attempting to cat-walk the slab across the chasm. The family hurries on, realizing the likely confrontation to come between Grumpy and Big Alice. The tyrannosaur stumbles but makes it into the allosaur's turf.

Dad assesses the layout of the Lost City plaza in order to determine the correct entrance mentioned in the diary. The central cave leads to Enik's place and the Sleestak pit; the near (right) entrance lies near an unmentioned fallen pillar. Dad concludes that it must be the one by the far side. In no time, Grumpy arrives, sending the Marshalls for cover between the fallen pillar's fragments as the battle between the two carnivores commences. Big Alice stumbles over the stone blocks during the showdown, dropping rubble onto the family and pressing the three to move on; our adventurers make a run for it and make it to the far opening. Dad's hunch was correct -- another Beware of Sleestak warning scrawled the passage wall confirms it. They leave the dueling beasts and continue on.

As the Marshalls quietly head through the haunted tunnels, they feel the cool air which keeps the Sleestak in hibernation. They arrive in a chamber containing gourds, a dusty old blanket, and a carron of rocks. Beneath the stones, they discover more of the diary, which includes a map. The author describes finding Harry Potts' trail, which he claims passes along "them dead lizard men" (? other episodes describe them as giant insects) and heads all the way back to New England. The family gets very excited, deducing from the abandoned artifacts in the chamber that the two men didn't come out the way they came in. Dad orients himself with the map, and they all head down a side passage.

While Grumpy and Big Alice fight it out, the family wanders quietly through halls lined with terrifying, webbed-in Sleestak (honest! they really look scary this time!) Holly is frightened at first, but Dad assures her that they're still dormant. The next room, encrusted with large glowing jewels, contains an arrow made of lined-up crystals pointing to a low crawlway lit by a dim orange glow. Apparently the one mentioned in the diary as leading back to Earth, the family anxiously crawls through.

When the Marshalls emerge from the passage, they discover a chamber holding an active lava pit. Holly soon sees the gruesome fate of the two soldiers: their clean, clothed skeletons lying face down on the ground. Will recovers the final segment of the diary, but before Dad can read it, he stops to notice the rising lava. The temperature quickly begins to rise; the family realizes the lava's role in bringing the Sleestak out of hibernation and rush out in a panic while they still can. Dad, Will, and Holly dash through the halls of the awakening Sleestak, barely escaping from the lizard (insect?) mens' clutches. At the exit to the plaza, they are held up by the still raging battle between the two mighty dinosaurs; in the struggle the Allosaur topples one of the few still-standing pillars. Dad and Will pull some crystals from their pockets, where they are tossed into the air and detonate. The blast frightens the carnivores enough to abandon their fight, and with the plaza cleared, the Marshalls make a run for home.

That evening around the campfire, Dad reads the remainder of the journal. It warns the reader to go back; this is not the way out of this God-forsaken land. The Sleestak awaken when the Devil's Cauldron bubbles up; he had stayed too long, with the Sleestak waiting for him to come out. The diary closes with the hope that his chum Harry Potts, whether in Heaven or in Hell, will put in a good word for his friend Peter Konik, a private in General Washington's Revolutionary Army.