The Marshalls are out one fine day practicing their swimming skills, specifically their ability to hold their breath under water. Holly surfaces to be congratulated by her father for staying under almost a minute. When Will gives it a shot, he unexpectedly surfaces in an eerie cavern. After returning to the jungle water hole, he relates the story to his family, speculating a connection between his new find and the Lost City. Though Holly finds the idea doubtful, Dad explains that the whole area is honeycombed with caverns and passages. Will is concerned that the Sleestak may come out through the waterhole into the jungle; Dad considers that risk worthy of investigation. He throws their clothes into a plastic bag from his backpack as they prepare to dive in to check it out.

The Marshalls surface in the pool of the spooky underground chamber, immediately noting the near-frigid temperature. Will peers down the passage leading off, wondering if it leads to the Lost City and/or Sleestak. They change into dry clothes before continuing on.

As they begin to explore the maze, Will, holding the torch and leading the way, is excited about investigating a side passage, but Dad holds him back, recommending the main tunnel on they way in. Meanwhile, Holly strolls along to discover a frightening band of web-covered Sleestak; the sight promptly invokes a terror-stricken shriek. Will responds defensively with his torch, but the Sleestak remain still. Holly asks if the monsters are dead, and Will says they look frozen, but Dad concludes that they are in hibernation (dormant Sleestak were mentioned and shown in previously aired episodes; this one must have been filmed earlier). He postulates that the Sleestak will awaken instinctively when the temperature rises, and urges that the family get back to their own cave, since these tunnels are probably full of the creatures. Will foolishly lags behind, curiously gazing at the dormant Sleestak until one of them awakens behind him. He turns to fend it off with the torch, but the two in front of him wake up as well. In the scuffle one of the monsters knocks the torch from Will's hand, but he manages to run off down a side passage. Down the main passage, Dad and Holly conclude that Will has been captured, so Dad orders his daughter back to their cave while he goes to rescue his son. Holly insists on going too, but the argument is interrupted when Sleestak come after them. Dad wrestles his way out of a Sleestak's grip, but the whole family has been split up, each member chased down a different passage by the goofy, galloping green giants (Sleestak can't run -- their feet are too big!) Holly does run back to the cavern with the pool connected to the jungle pond, but a Sleestak follows her. Even after she climbs out onto the shore in the jungle, the creature still pursues; she drives it back underwater with a burst of light from some crystals.

Dad is lost in the maze of water-scalloped tunnels; thinking to himself, he decides to use a suspiciously soft, chalk-white rock off the floor to mark the wall of each visited passage. Meanwhile, Holly waits in worry for Will and Dad, only to be driven away from the waterhole when Spot, Spike and Emily head on over for a drink. Back in the cavern, Will is relentlessly pursued by the Sleestak; they have chased him into the Lost City's tunnels. Surrounded, he falls backward into a room whereupon he draws his knife to fend them off; to Will's surprise, Enik appears, assuring him that his weapon will not be necessary.

Enik assures Will that the Sleestak cannot harm him; with the wave of a hand, the Altrusian invokes a mist creating a non-shocking force field in the doorway. When Will voices his surprise upon finding the Altrusian still around, Enik begins to explain that he is unable to leave, and leads his guest toward the matrix table to demonstrate.

With the dinosaurs gone, Holly dives back into the jungle pond to return to the underground caverns. Dad, still lost in the tunnels, discovers he has gone in circles when he sees one of his chalk marks on the passage wall. In response to Will's questions, Enik explains how though he has been able to open the dimensional doorway to his own time several times, he has not been able to pass through. Enik's technical description of the problem (the apparent reversal of the law of temporal momentum resulting in considerable temporal pressure) baffles Will Marshall, so the Altrusian apologizes for Will's lack of knowledge on ultradimensional matrices. He restates the principle in simple terms: journeys through the time doorway require balanced exchanges of equal mass and temporal energy between the two worlds. He goes on to demonstrate that the time portal is locked on the events surrounding the Marshalls' transfer into the Land (which look suspiciously like the show's opening credits). Will inexplicably notes the absence of "time doorway" mist below their falling raft (there is plenty of condensing water vapor -- can he even tell the difference?) and concludes that the family was actually killed after tumbling over the falls. When he questions Enik to explain the paradox, the Altrusian simply states that the Marshalls should not have entered at all, and must somehow be removed before the workings of the time doorway can be restored.

Deeper in the maze of tunnels, Holly wanders about searching for her family when she hears something approaching. She crouches down near a boulder to hide, but the thing reaches down to touch her. Luckily it is only her father; he comforts his startled daughter with a hug. Dad warns Holly that she should have stayed out in the jungle, though Holly explains that she still had to fend off a pursuant Sleestak with some light crystals, and wanted to get back with Dad and Will. It is then that she realizes Will is still missing. Dad believes his son escaped down the main passage, in which Holly finds their extinguished torch; though at first Dad considers its use too dangerous, possibly awakening more Sleestak, Holly whimpers about the cold, still wearing her wet clothes, so Dad lights it up, noting the Sleestaks' aversion to fire.

Back in the dimensional doorway chamber, Will and Enik continue to ponder the paradox. The Altrusian believes that the family could have only experienced either getting transported to their present location, or dying in the crash upon the rocks below the falls. Yet somehow, neither event happened; they did not reach the bottom nor did they enter the time vortex. They seemed to have fallen into space. Enik continues to respond to Will's probing about their undeniable presence here and how by restating their existence as inappropriate and a hindrance to his own return home.

While searching the passages for their missing kin, Holly spots a light ahead. Though Dad advises they be on guard, he is grabbed by a sneaky Sleestak who was able to hold back its hiss long enough to gain surprise. Holly temporarily drives it back with the dropped torch and quickly hands it back to her father; the Sleestak, however, aggressively harasses them, prompting Dad to order Holly into the lit room off to the side. Try as she might, Holly is unable to enter the chamber until Will notices his family's arrival and requests Enik to momentarily bring down the invisible barrier.

Holly and her father stumble into the room, where they are assured that the Sleestak (dancing around in a corny pantomime) cannot penetrate the force field. Enik asks for Dad's assistance, but before he can explain, they all watch the repeating sequence of events through the time doorway. Holly remarks that they have already tumbled over the falls to arrive in the present world, but Will corrects her, quoting the Altrusian's assessment that the Marshalls never fell through the doorway. Dad considers such a situation impossible; Enik identifies it as the source of the paradox that has locked the matrix and rendered it useless. Dad then proposes that the family could throw a rope through the doorway to reach the falls and thus Earth; the Altrusian points out the that the rope would be severed as soon as the rapid sequence starts over. Enik further confuses the Marshalls by incorrectly depicting the paradox as the family being both at the falls and in the dimensional doorway chamber "at the same time". When Enik proposes his solution, which is to prevent the family from being transported through in the first place, Holly voices her objection -- as an inevitable consequence, they would plunge to their deaths! The Altrusian sarcastically compliments her brilliant realization, but Dad brings to light a flaw: they already exist in the here and now, which cannot be undone. Dad goes on to reveal that he saw the time portal open below them, golden and misty; it remains unclear, however, what transpired after that, or if they even passed through the doorway at all.

Enik is at a loss to explain, and repeats for Rick Marshall that three people of equal mass must enter for them to leave. Dad ingeniously volunteers the three people tumbling over the falls, and entices the Altrusian with the benefit of the exchange restoring the doorway to permit his own journey home. Enik, however, faces an ethical dilemma, since doing so would violate Altrusian code of law. Dad's nagging reveals that Enik will instruct Dad on how to proceed; with his own civil integrity protected, the Altrusian hesitantly agrees. Enik orders Dad to place a white crystal from the wall onto the matrix; the time doorway will open, the raft will fall in. After it is done, Enik (prematurely) congratulates Rick Marshall for resolving the paradox; when Dad asks if they can now return home, the Altrusian emphasizes its necessary completion. The raft remains suspended between the two worlds until the local Marshalls enter the portal; to complete the resolution of the paradox, the family must go home.

Overjoyed, the family stampedes toward the time doorway, but Holly stops to bid their friend goodbye. After Will and Dad bid farewell, Holly thanks Enik with a kiss on the cheek. The earthly custom baffles him; Dad then explains the significance of the human gesture. Enik admits to his race's minimal expressiveness (give him a break, his face is hard as rock), but does return the gesture with the only Altrusian word we ever learn: with outstretched arms, his hands upon Holly's shoulders, he sincerely replies "Ginactik". Translated, it means good health, good life, and go in peace.

Will hurries Holly along as the Marshalls pass through the time portal. What follows are the sequences after the family landed with their raft near the swamp. The usual opening sequence is played, with additional footage after they take refuge in the cave from Grumpy. After the tyrannosaurus gives up, Will and Holly poke around in the cave. Dad declares it their new home until they find their way back, which he promises they will do someday. Enik, having watched the entire sequence through the doorway, declares the cycle complete and once more bids his friends farewell.