Plot
The founder of "quantum paleontology", Dr. Rick Marshall has a low-level job at the La Brea Tar Pits, three years after a disastrous interview with Matt Lauer on Today went viral and ruined his career. Doctoral candidate from Cambridge Holly Cantrell tells him that his controversial theories, combining time warps and paleontology, inspired her.
She shows him a fossil with an imprint of a cigarette lighter that he recognizes as his own, along with a crystal made into a necklace that gives off strong tachyon energy. She convinces him to finish his tachyon amplifier and go on an expedition to the Devil's Canyon Mystery Cave theme park where she found the fossil.
As they float into the cave on a small inflatable raft with the theme park's owner Will Stanton in the role of a paddler and narrator, Rick detects high levels of tachyons. Activating the tachyon amplifier creates an earth-shaking time warp the raft falls into. Having regained their senses in a sandy desert interspersed with items from many eras and discovered that the amplifier is nowhere to be seen, the three travellers rescue an apeman by the name of Chaka, who becomes their friend and guide.
They spend the night in a cave where they have taken refuge from a pursuing telepathically endowed tyrannosaurs Rex they nickname "Grumpy", who develops a grudge against Rick for being intellectually disparaged. In the morning, Rick receives a telepathic invocation for help and is being drawn to run towards ancient ruins, where they encounter reptiloids called the Sleestak before meeting Enik the Altrusian, who sent the message. Exiled by the Zarn, who wants to take over the Earth with his Sleestak minions, Enik can prevent the invasion if Rick gets the tachyon amplifier.
Led by Chaka, the group enters a rocky wasteland littered with artefacts from different epochs, encountering compsognathuses, dromaeosaurs, Grumpy, and a female allosaur nicknamed "Big Alice". These last two are battling it out over the remains of an ice-cream seller killed by the dromaeosaurs, until they sense Rick and pursue him. Rick kills Big Alice with liquid nitrogen, finding the amplifier was swallowed by the allosaur. But a pteranodon snatches the amplifier into a volcanic crater functioning as an incubator. Treading lightly on the thin volcanic-glass floor of the glowing crater, Rick gives himself over to the music of A Chorus Line coming from the tachyon amplifier, and dancingly meanders between the pterosaur eggs towards the device. When he reaches it, the playback suddenly stops. The eggs begin to hatch, and they realize that the music was keeping the baby pterosaurs asleep. Rick, Will and Holly belt out "I Hope I Get It", with Chaka joining in to display a great singing voice, much to everyone's surprise.
While Rick, Will and Chaka go on a psychedelic spree, Holly is walking about with the tachyon amplifier, which detects a signal coming from an underground cavity, where she picks up a dinosaur egg and learns from a holographic recording left by the long-deceased Zarn that Enik the Altrusian is an escaped convict who, having overtaken the central pylon and its tachyonic crystals, is planning to go on a rampage across time and space. The recording ends with the Zarn being killed by Enik. Deemed to be guilty of providing assistance to Enik, she is kidnapped by Sleestak and brought to the Library of Skulls for a summary execution. Having sent Chaka to bring Enik, Rick and Will rescue Holly by pushing the Sleestak executioners into a well with red-hot magma at its bottom. Enik arrives with a tachyonic crystal, whose power allows him to establish telepathic control over the remaining Sleestak, one of whom is carrying the tachyon amplifier. The villain leaves to open a wormhole between the prehistoric Land of the Lost and the modern Earth, which is the first habitable planet to be pervaded by his army of rapidly reproducing Sleestak reptiloids.
At that point, Grumpy appears to settle the old score with Rick, who pole-vaults into Grumpy's maw and, by dislodging an intestinal obstruction, earns the beast's gratitude. Riding atop the tyrannosaur, he joins the others to defeat the Sleestak army and confront Enik. After one of the crystals sustaining the wormhole is shattered, Enik reveals that the portal will close forever. Rick grabs Holly's crystal and inserts it into the vacant socket, but it becomes apparent that the substitute crystal will not hold for long. Enik catches Will by an ankle to prevent Rick's departure. With the help of Chaka, Will restrains Enik and chooses to stay. Marshall and Holly leap into the portal, whereas Will is later welcomed by a bevy of cheerful and attractive girls of Chaka's tribe.
A triumphant Rick reappears on Today with the dinosaur egg Holly brought back, promoting his new book, Matt Lauer Can Suck It!, causing Lauer to start attacking him. Left behind on the Today set, the egg hatches a baby Sleestak, which hisses as the screen goes black.
