During the First Ice Age, a caveman and cavewoman who are partners attempt to make fire when an earthquake causes a cave-in that buries both of them.In 1992 Los Angeles, another earthquake awakens Dave Morgan (Sean Astin), an average teenager who, together with his best friend Stanley "Stoney" Brown (Pauly Shore), is striving to attain popularity in high school, but comes off more like an outcast or reject. He is in love with Robyn Sweeney (Megan Ward), an attractive and sweet girl whom he first befriended at grade school and rejected on several occasions before she reached adolescence. Her boyfriend is Matt Wilson (Michael DeLuise), a bully and jock who constantly humiliates Dave and Stoney, often directly as a result of Dave's affections towards Robyn.One day, Dave is digging in his backyard preparing to install a new swimming pool when he finds a block of ice with the body of a man (Brendan Fraser) inside it. He leaves the chunk of ice in his garage while he goes away. Space heaters left on inside the garage cause the ice to melt and set the caveman free.When Dave returns home with Stoney, they find that the ice block has disappeared, there are hand prints all over the walls and the house is in disarray. They hear a smoke alarm beeping and set off to investigate, finding the caveman inside Dave's bedroom trying to start a fire. The sight of Dave and Stoney makes the caveman start to panic, as does the sound of a telephone ringing, but Stoney is eventually able to calm the caveman down by mesmerizing him with the flame of a lighter. Dave and Stoney bathe the caveman and trim his hair to look like another average teenager, after which Dave names the caveman Link, regarding him as being the "missing link".Dave and Stoney are able to get some clothes for Link and to fool Dave's parents and sister into thinking he is actually an Estonian exchange student named Linkavitch Chomofsky who has been sent to live with them. Dave and Stoney enroll Link at their high school, where his bizarre behavior and supreme athletic abilities make him quickly become popular, and Dave and Stoney are also shot to popularity by their association with Link, which allows Dave to get closer to Robyn, causing Matt to become frustrated and angry. Before too long, Stoney's bizarre attitudes have effects on Link's actions and speech, causing a rift to develop between Dave and Stoney. Matt starts a fight with Link at a skating rink, but when Robyn grows attracted towards Link, Matt's frustration and anger increase.The school go on a field trip to the La Brea Tar Pits, where Link becomes upset when he realizes that the other cavepeople that he knew are all dead. Dave and Stoney console Link by telling him that he isn't without friends in this time, leading to the trio making a pact. During a Driver's Education class, Link drives away in a car in which Dave, Stoney and Robyn are all passengers. After the car stops at a dance club, Dave and Link are both arrested. Dave tries to abandon Link, having had enough of his shenanigans and upset that Robyn has chosen to take Link to the upcoming prom, and ends up getting into a fight with Stoney, which leads Link to return and break up the fight. After they are separated by Link, Dave and Stoney reconcile.Link, with Robyn as his date, proves to be a hit at the prom night party, while a dateless Dave stays at his home. Matt breaks into Dave's bedroom and steals some photos that prove that Link is a caveman. Matt and his friends take the photos to the prom, and Dave and Stoney pursue them until another earthquake strikes. At the prom, Matt shows the photos and exposes Link as a caveman, but the new information makes Link even more popular, causing Matt's plan to backfire and leave him humiliated. Dave makes up with Robyn before he, Stoney and Link lead the whole prom in an impromptu caveman-like dance, for which the music is provided by Infectious Grooves.When the prom is over, Dave brings some of the students to his house for a pool party where Dave and Robyn seal their reconciled relationship with a kiss. Meanwhile, Stoney and Link discover some breast prints on a slider and paint covering the walls, clues that are similar to the ones that Link left before Dave and Stoney first found him. They follow some muddy footprints into the bathroom and discover a beautiful cavewoman in the bathtub, who turns out to be Link's partner seen at the beginning of the film. Link joins her in the bathtub as Stoney happily embraces the cavewoman and cheers her and Link on. After she finishes bathing, Stoney and Link make her over to resemble a modern woman.