Three years after being expelled from the science department due to his controversial theory on Time warps and a misguided attempt on Matt Lauer's life, Rick Marshall finds himself teaching at the George C. Page Museum. His mundane life takes a thrilling turn when he meets Holly Cantrell, whose discovery inspires him to build a tachyon amplifier for time warp travel. They decide to explore the Devils Cave attraction, recruiting the tour guide, Will Stanton, to assist them. However, their experiment quickly spirals out of control, resulting in their entrapment within a bizarre parallel universe where the timelines collide.
As they navigate this strange new world, they encounter a group of ape men known as Pakuni. When two of the apes threaten a younger member, Will Stanton inadvertently teaches them how to create fire, leading to an unexpected alliance. They soon encounter Cha-Ka, the young ape who joins them on their adventure. Their journey takes a perilous turn when they stumble onto a dinosaur feeding area, where they are pursued by a terrifying Tyrannosaurus rex.
Rick, confident in his understanding of dinosaur behavior, foolishly asserts that the T. rex cannot judge depth due to its small brain. To their shock, the dinosaur proves him wrong, jumping across a chasm in pursuit. The group narrowly escapes but finds themselves facing the relentless creature, which Rick nicknames "Grumpy." Holly soon realizes that the T. rex's interest in them is personal when it leaves behind a "gift" wrapped in the form of a walnut.
The next day unfolds with more challenges, as Cha-Ka arrives with fruit infested with spiders. In an odd attempt to camouflage himself, Rick covers himself in dinosaur urine to mask his scent from Grumpy. This bizarre tactic leads to visions of a creature in distress, prompting the group to venture out into the mystical Lost City. Here, they encounter the Sleestaks, a slow but lethal threat, and a mysterious being named Enik, who reveals the nefarious plans of the villain Zarn seeking control over the universe and desiring their tachyon amplifier.
Despite Will Stanton's reservations, Rick offers to help; together with Cha-Ka, they traverse a landscape filled with deadly dinosaurs and the chaotic presence of the Compsognathus and Velociraptors. A dramatic encounter ensues when Big Alice, an imposing Allosaurus, chases them down, forcing Rick to use his quick wits – and a tank of liquid nitrogen – to freeze the dinosaur.
After some comical misfortunes and an overheard sting from a giant mosquito, Rick regains his confidence and attempts to unite his team through a song, only to be distracted by the chaos that ensues. Holly uncovers a Pylon and realizes a hidden plot within, leading to her capture amidst a trial conducted by the vicious Sleestaks.
Meanwhile, the group confronts their own challenges, from disguises to fruitless searches for Holly, culminating in a gripping showdown with Enik, who reveals his true villainous intentions. As the situation intensifies, Rick fully embraces his fate, facing Grumpy in a one-on-one duel. Through a series of misadventures, they end up forming an unlikely truce. Together, they storm the Pylon, destroy its power source, and secure their escape with a heartwarming yet hilarious resolution.
Ultimately, with the tachyon amplifier in hand and lessons learned, Rick returns to Earth, where he relishes in his newfound fame. He challenges Matt Lauer with a signed book titled Matt Lauer Can Suck It, but not without irony; history seems poised to repeat itself as chaos ensues once again with the hatch of a mysterious egg, hinting at more adventures to come.
