Early one morning, Will is awakened by an eerie sound. He steps outside to investigate, only to be unintentionally startled by Dad. Will asks his father if he had heard anything strange, but gets a negative reply. Will tries to explain what he had heard, but is interrupted by Holly's griping that some animal has gnawed at their sack of food for the third night in a row. Dad suggests that their little night visitor could have been the source of the noise, which Will dishonestly agrees with. Dad then assigns Holly to the project of designing and building a trap to catch the animal, and with a bit of advice from her father, she plans to create one using a hollow gourd with carrots for bait. On a roll of assigning chores, Dad next hints at his son to weed the garden.

While taking a break from weeding, Will again hears the trembling, hypnotic tone, falls into a trance, and wanders over to the Lost City. Within a large chamber, he finds what appears to be a very crude time-doorway control table, with a pulsating light-jewel nearby. Upon holding the gemstone, an empty alcove fills with mist; a wavery image of his deceased mother comes into view.

Back at the cave, Dad startles Holly, who is testing out her trap, when he asks her of Will's whereabouts. Neither of them know where he has gone, so they start looking for him out in the jungle. After searching and calling out his name for a while, Will strolls over to them, innocently claiming that he just went for a walk. As the family heads back to High Bluff for dinner, Will begins to tell his father, but strangely decides to forget about the matter.

The next morning, when Holly grumbles about her trap not working, Dad enlightens her with the wisdom that the bait must be the only food around in order to tempt and catch their little night visitor. After building a bamboo frame from which to hang the food sack safely out of reach, Holly calls out to her brother working in the jungle to come see her handiwork, but once again Will wanders off under a strange spell. She follows him to the Lost City, finding him holding the flashing crystal and staring into an empty alcove. She nudges at her brother, shaking the jewel from his hands, awakening him from the trance. Will is reluctant to tell her sister what he has seen, so Holly impatiently finds out herself by picking up the glowing stone. Will joins in the vision by touching the stone, but both children resist the sweet calls of their mother for her children to come home to her. The apparition soon fades, inviting them to return the next day but asks to keep this a secret. Holly recommends they tell Dad; Will warns that he has tried, but somehow can't.

After dinner, the kids sit around the table, listless and withdrawn. Dad's probing sends an irritable Will storming out of the cave; he also gets a minimal response from Holly, who tries unsuccessfully to tell him about the discovery. Will apologizes, pressuring Dad to reluctantly dismiss his childrens' strange behavior.

The following morning while Holly helps Will tend the garden, the kids discuss the possibility of having peered into the dimensional doorway to a time when their mother was still alive, but mutually state there desire not to return to that spooky place. Dad drops by to check on them, but another attempt by Holly to confess to her father fails.

Near sunset, just as the siblings finish their gardening, the entrancing sound returns, rendering them helpless under its power. Dad, hiding not far away, spots them wandering off and trails them to the Lost City. Gazing into the mysterious mist, Will and Holly resist the calls of their mother's phantom, but are successfully enticed into the doorway when their father's image materializes. The shocking truth unfolds when surreal flashes of the actual events are shown -- hypnotized Will and Holly are being led to the pit by a towering Sleestak! Not far behind, Dad strays into the false time doorway chamber and picks up the blinking crystal. When he sees Will and Holly in the mist, enticing him to join them, he denounces the reality of the vision; he thrusts the jewel to the stone floor, where its explosion frees him from the spell. A Sleestak is revealed after Dad's return to reality, sending him chasing after it into the pit room to rescue his children.

For a brief moment, Will and Holly see two Rick Marshalls in the room, until their real father knocks the hypnotic gem from their hands. Dad grapples with one Sleestak, shoving it back against the far wall, but another closes in; he saves the day by grabbing the entrancing jewel from the ground and shattering it against the adjacent wall, where it disintegrates in a blinding burst of light, letting the Marshalls escape.

Gathered around the safety of their campfire, the family discusses the whole ordeal. Dad hypothesizes that the blue crystals hypnotized them into seeing what they loved most, and that the Sleestak passed up the opportunity to take Will the first time for a chance to capture them all. Holly takes a dislike to traps, and destroys the one she had made.