Pompous paleontologist Rick Marshall has a low-level job at the La Brea Tar Pits, three years after a disastrous interview with Matt Lauer of Today became a viral video and ruined his career. Doctoral candidate student 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 come help her on a seemingly routine expedition to the cave where Holly found the fossil, which is in the middle of nowhere. With cave gift shop owner Will Stanton they raft into the cave, where Marshall has detected high levels of tachyons. He activates the tachyon amplifier, triggering an earthquake that opens a time warp into which the raft falls. The group finds themselves in a desert, filled with various items from many eras, and without the amplifier. They rescue a primate-like creature, Cha-Ka of the Pakuni tribe, who becomes their friend and guide.
The gang spends a night in a cave after surviving a meeting with a fast, intelligent Tyrannosaurus they nickname "Grumpy", who stalks Marshall for calling him stupid. Marshall receives a telepathic message begging for help and finds himself in ancient ruins. The group encounters a race of lizard men called Sleestaks before meeting the one who sent Marshall the telepathic message, Enik the Altrusian. He explains that he was exiled by the evil Zarn who is attempting to take over Earth with his Sleestak minions, but Enik can prevent this if Marshall retrieves the tachyon amplifier.
The group stumble upon a desert where many things from across time end up and they encounter many Compsognathus, Dromaeosaurs, Grumpy, and a female Allosaurus. The Allosaurus and Grumpy battle it out over a previously killed ice-cream seller until they sense Marshall and chase him. Marshall kills the Allosaurus with liquid nitrogen and finds that the amplifier was eaten by the Allosaurus. The amplifier is then stolen by a Pteranodon and taken to its nest. The group arrives at the nest and Marshall lightly steps through the Pteranodon eggs to retrieve the amplifier, but when he reaches it, it stops broadcasting the soundtrack to Marshall's favorite musical A Chorus Line. When the eggs begin to hatch, Holly realizes that the music was acting as a sort of lullaby keeping the baby Pteranodons asleep. Marshall, Will and Holly belt out "I Hope I Get It", with Cha-ka inexplicably joining in, displaying an impressive singing voice.
Marshall, Will and Cha-ka celebrate their good fortune. Meanwhile, Holly pockets a dinosaur egg and learns from a recording left by the long-deceased Zarn that Enik deceived them and he is actually the one planning to invade Earth. But she is captured by the Sleestaks to be brought to the Library of Skulls for judgment. The others saved her from being executed for helping Enik, but the villainânow possessing the amplifier, and mind-controlling the Sleestaksâleaves them to open a portal to Earth. Marshall pole vaults into Grumpy's mouth and after removing an intestinal blockage, befriends him. He joins the others to defeat the Sleestak army and confront Enik. After the crystal link between the Land of the Lost and Earth is shattered, Enik reveals the portal will close forever. Thinking fast, Marshall grabs Holly's crystal and inserts it into the port, knowing that the substitute crystal won't hold for long. Will chooses to stay behind to live a better life and to prevent Enik from following Marshall and Holly back to Earth, learning later that female Pakuni are very attractive.
A triumphant Marshall again appears on Today with the dinosaur egg Holly brought back to promote his new book Matt Lauer Can Suck It. The egg left behind on the Today set hatches a baby Sleestak, which hisses as the screen goes black.
Whay song did Marshall, Will and Holly start to sing?
I Hope I Get It