Masten Thrust Jr. is a wealthy big-game hunter who runs Thrust Inc., a multimillion-dollar company that drills for oil under the polar caps with a manned laser drill called the "Polar Borer." After one expedition, only geologist Chuck Wade returns, recounting that the crew went through the icecap on a routine check and surfaced in a volcanic valley. There, while the others explore, they are slain by a Tyrannosauruserm; Wade’s story convinces Thrust to go after the creature himself. He gathers a small party: Bunta, a Maasai tracker; Dr. Kawamoto; and Francesca Banks, a Pulitzer Prize–winning photographer chosen by the press pool. Though Thrust is initially reluctant to bring Frankie along, she wins him over with persistence and a bit of charm.
Using the Polar Borer to reach the valley, the group spots Pteranodons and raft to shore, narrowly avoiding a stampede from a Uintatherium. After setting up camp, Thrust, Wade, Bunta, and Frankie venture out in search of the T. rex, while Kawamoto stays behind. They soon confront the enormous predator and manage to escape its first assault. The beast later finds the camp, destroys it, and murders Kawamoto before dragging away the Polar Borer. With the Borer gone, the dinosaur digs a canyon in a wall of bone and releases a Triceratops, which clashes with the T. rex; in a brutal confrontation, the Tyrannosaurus rex kills the Triceratops and claims dominance.
Returning to the burned-out camp, the party discovers Kawamoto and the Polar Borer missing. Thrust’s rage grows, and he vows to kill the dinosaur. Months pass, and the survivors relocate to a cave, where they encounter a number of cavemen. They manage to fend off the intruders with a handmade crossbow and soon befriend a cavewoman, Hazel, who helps Frankie with washing her hair. The T. rex returns, forcing Frankie to seek shelter in a cave while Maston, Bunta, and Chuck fend off the assault with a heavy boulder tied to the creature’s tail. Realizing they need a decisive weapon, Thrust builds a catapult to bring the monster down.
During another hunt, Chuck discovers the Polar Borer is still operable, but Maston refuses to leave until the beast is dead. Chuck and Frankie depart to repair the Borer and depart with it, leaving Maston and Bunta behind. When the Borer is launched back into the water, Frankie returns to urge the others to leave one final time. Tragedy strikes as Bunta is killed by the T. rex while stalking him. Frankie reunites with Maston and helps him use the catapult on the dinosaur, but the beast rises again and shatters the device.
As Wade arrives with a warning that they must escape the valley or be trapped, Frankie pleads with Maston to depart with them, arguing the T. rex is the “last one.” Maston refuses, replying with a quiet resolve: > So am I. Wade and Frankie then depart aboard the Polar Borer, leaving Maston to face the valley alongside Hazel.
