The movie opens with the 'Hostile' Tribe, led by Tonda (John Matuszak), who is the largest and strongest, therefore the Alpha male, all foraging for food. We then see the Omega male, Atouk (Ringo Starr), the smallest and least powerful, being pushed and bullied from tree to tree as he tries to find some food for himself. He is about to grab a small lizard when he sees a shadow behind him-- a much bigger lizard-- a dinosaur. He runs back, shouting to his tribe about the attack. They scatter and climb trees, trying to get away. The dinosaur pushes one tree and knocks it over, dislodging the cavemen at the top. It grabs one of the cavemen, Noota, by the leg and drags him off to make a meal of him.
The cavemen regroup, all blaming Atouk for the loss of Noota and there being no more food to bring home to their cave. As they start to head back, Atouk notices his friend Lar (Dennis Quaid), who tried to fight the dinosaur by jumping on its tail, has twisted his leg and injured it when the dinosaur threw him off. The tribe's Beta male, Ock (Avery Schreiber) notices Atouk helping Lar develop a limping walk and alerts Tonda. Ock and Tonda quickly deduce Lar is "Pooka" (hurt or 'broken') and drag Atouk away, essentially abandoning Lar.
The tribe arrives back at its cave where Ock tells the cavewomen about the dinosaur attack and the loss of Noota, which Noota's mate takes pretty hard. Ock reminds them all that it's Atouk's fault that Noota is gone and the tribe could not bring back any food. Atouk dejectedly sits by his corner of the cave where he's made drawings. He picks up a roughly circular flat stone that has gotten a stick pushed through a hole in its center. Atouk twirls the stone around (a reference to the wheel) and tosses it aside. He pulls a small fruit from a hidden niche, and is about to eat it, when he notices Tonda's mate, Lana (Barbara Bach), the most beautiful of the cavewomen, and who Atouk is secretly in love with. Lana is dejected because she is hungry and the tribe had no food to bring back. Atouk gives Lana the fruit. She is grateful for the fruit, but Atouk is upset when she gives it to Tonda.
Later the tribe's men are out gathering more food. They come across a tree laden with berries that they find suspicious. They grab Atouk to use as a taste-tester. As he eats one of the berries he finds that it is making him euphoric and sleepy. Before he passes out, he secretes a couple of the berries in his clothing. The cavemen see Atouk pass out, decide the berries are not suitable as food, and pick Atouk up and drag him back to the tribe's cave.
Back at the cave, Atouk takes another fruit out of his hidden cache. He pulls off the top and scoops out the inside, filling it with the small berries that put him to sleep. He presents the fruit as another offering to Lana, figuring that she will give it to Tonda, and Tonda will fall asleep, and then Atouk can make a move on Lana. But when Atouk's back is turned, Lana also eats some of the fruit and both she and Tonda fall deeply asleep.
Late at night, Atouk tries to carry Lana off to make love to her, but her body is too rigid. Finally he simply puts an arm around her and falls asleep. As dawn starts to rise, Tonda turns over and cuddles what he thinks is Lana. Atouk tries to get away but Tonda's strength is too strong. Tonda gradually awakens and realizes what Atouk was up to. Furious, he chases Atouk out of the cave, essentially banishing him from the 'Hostile' tribe.
Atouk wanders off alone into the wilderness, but finds a set of footprints... a single foot making tracks heading off. He realizes this must be Lar and rushes in the direction of the tracks. He finds Lar and calls out to him. Atouk copies Lar's limping hop as they rush toward each other to hug. The force of the hug straightens out their arched spines, making them able to stand straighter, taller... and Lar can walk normally again without limping.
That night, Atouk is still mooning over Lana, drawing a little stick figure with enormous breasts in the dirt. However, Lar, with a combination of drawing (a stick figure with an enormous nose, representing Atouk) and gestures (his fist, representing Tonda), reminds him that Tonda is too strong. After they go to sleep, an enormous prehistoric insect lands in Lar's face. Atouk wakes up and smashes the insect on his friend's head and simply goes back to sleep.
Atouk and Lar are passing by a tar pit when they find a young blonde woman (Shelley Long) leading an old man (Jack Gilford) who walks with the aid of a stick. She tries to turn him away from the tar, but soon as she turns back around he steps forward again and starts to sink into the tar. Atouk and Lar scramble to rescue him. Atouk and Lar introduce themselves, and the women introduces herself-- Tala-- and the old man-- Gog. Tala shows that Gog is 'pooka' (specifically, he is blind). Atouk and Lar straighten Gog's and Tala's spines out. Tala keeps turning back to face Atouk every time he tries to turn her back to him so he can straighten her spine. finally he grabs her face to face for a bear hug that straightens her posture. Right away she looks at him in a way that shows she likes him.
Gog gets separated from the three after they bathe in a large river to wash off the tar. Gog bumps into what he thinks is a cliff, but feeling it with his hands, sees it is softer and smoother than rock. The camera pans out to show that Gog is masturbating a large tyrannosaur-like dinosaur. Puzzled, he whacks the 'cliff' with his walking stick, essentially bashing the dinosaur in the nuts. It roars in pain and Atouk, Lar and Tala realize a macha is nearby. It turns and spots them, chasing them along a cliff and cornering them on a ledge. As it closes in, Atouk struggles to lift a large tree branch. The point sticks in the dinosaur's chest, and halts its attack. Gog finds his way back to them and they celebrate their escape, Tala hugging Atouk repeatedly. Lar notices and thinks that Tala should be Atouk's girlfriend. Tala seems to like the idea, but Atouk reminds him that he is in love with Lana. His victory over the dinosaur makes Atouk think he can now fight Tonda. He grabs Gog's stick and sets off for the Hostile Tribe cave... Tala repeating Lana's name in a tone that hints at jealousy.
Atouk goes to the cave to confront Tonda. Noota's widow bluntly tells Atouk that the tribe's men have gone out to gather more food. Atouk asks about Lana and Noota's widow silently gestures toward her. Lana is impressed at Atouk's straighter posture and greater height. But Tonda and the tribe's men return, and suddenly Atouk's resolve breaks and he sneaks out of the cave through another opening as the tribe's men enter. Outside, Tala smugly mocks Atouk, but he quiets her down.
Late at night, Atouk and Lar sneak into the cave to kidnap Lana. They pick her up and Atouk presses his hand tightly over Lana's mouth to keep her from screaming. As Atouk and Lar exit the cave, however, a jealous Tala grabs Atouk's hand and pulls it away from Lana's mouth. Right away Lana begins to scream for help. Tonda awakens and chases Atouk away again.
Atouk, Lar, Tala and Gog are sleeping near a small cave when another caveman comes up and looks at them. Seeing them awake, he dashes into the small cave. Atouk awakens and realizes the caveman was there. He goes to the cave entrance and calls him out, saying he is a friend. The other caveman sticks his hand out and he and Atouk give a friend's handshake. The man, Folg, exits the cave, accompanied by his family.
Gog and Tala discover a melon vine right nearby... but the quadrupedal dinosaur is munching on the melons. It spots Tala and starts to move toward all of the cavepeople. Gog, not yet realizing the 'macha' is there, tries to give the melon in his hands to Atouk. As the dinosaur advances, Atouk throws the melon at it-- and it sticks right onto the horn atop the dinosaur's nose. The melon interferes with the dinosaur's vision and it cannot shake the melon off. Frustrated and its vision crossed and unsteady, the dinosaur stumbles off as the new Misfit Tribe celebrates. Lar straightens out the spines of Folg and his family and they all head off again.
A rainstorm breaks out that night, and the Misfits seek shelter inside the hollow of a large tree. Lightning breaks off a large piece of tree branch and it falls to the ground, burning. The Misfits, curious, step toward the burning branch and finds that the fire warms and comforts them, though Atouk discovers if he touches the fire, it hurts ('pooka,' he tells them). Atouk names the fire 'haraka.'
Other eyes are watching the Misfits. Atouk and Lar realize they are being watched and call out, "Bobo.' The newcomers come out of hiding and more introductions are made-- the newcomers include Bork and Kalta, the leaders; a dwarf named Ta, a man named Nook who carries an iguana, and two men, Ruck and Flok, who are same-sex lovers. An amusing dialogue follows with Nook (who speaks perfect modern English, the only character in the movie that does so), translating the various 'caveman words.'
Bork and Kalta argue over a large birdlike creature they've killed. Remembering seeing Tonda breaking a huge haunch of meat in half, Atouk tries to settle the argument by breaking the bird in half. Struggling with the bird, he winds up holding it over the fire. The fire begins to cook the bird and everyone starts sniffing the air, noticing the wonderful smell. The newly expanded tribe enjoys a hearty meal around a campfire. Ta drinks from a gourd and sighs with satisfaction, inadvertently blowing into the gourd. The air makes a pleasant-sounding noise that amuses the tribe. Ta starts to blow into the gourd with a steady rhythm. The tribe members quickly pick up bone pieces and stones, picking out a musical tune and creating man's first band session, jamming into the night.
Morning comes, and the tyrannosaur comes across the sleeping Misfit tribe. It advances, looking for a meal of its own. As the tribe scatters, Atouk spots the smoldering campfire and calls out to the dinosaur, "Macha! Macha!" The dinosaur rushes toward Atouk and steps right onto the campfire. We see the dinosaur begin to sniff the air, and suddenly begins to howl in pain as it realizes its foot is burning. It rushes off, leaving the tribe safe and triumphant over another dinosaur attack.
The tribe realizes they're out of food. Atouk gathers the tribe's men and they go off to hunt and forage. The wilderness is largely barren (apart from a large muddy spot which amusingly turns out to be dinosaur poop!), and the tribesmen begin to notice Nook's iguana. They are starting to consider grabbing it and making a meal of it when Nook holds it protectively and shows the tribe what the iguana can do... he sets it on the ground and it 'points' toward a large pterodactyl egg. The Misfits begin to carry the egg back to their camp.
The egg is heavy and the Misfits must put it down to rest a bit. Suddenly Tonda arrives, leading the Hostile tribe as they are also out looking for food. They see the egg and move toward it. As they begin to circle around the egg, Tonda spots Atouk and an amusing chase begins around the egg, until the Misfits give the Hostiles the slip and hide behind a rock.
The Hostiles begin carrying off the egg, but the pterodactyl arrives and dives at them, trying to recover it. Trying to escape, the Hostiles accidentally drop the egg off of a cliff, where it falls into a small hot spring and breaks open. The Hostiles think the egg is ruined and slink off, while it cooks on top of the spring, resembling a sunnyside-up egg.
The Misfits find their way to the egg and find that the hot spring has cooked it to perfection-- it is not ruined but delicious. They gather up the egg in chunks and bring it back to their camp.
The Misfits are about to enjoy another meal when Tonda and the Hostiles find their way to the Misfits camp and try to raid it. The Misfits grab sticks and make torches, using them to keep the Hostiles at bay. Grot and Ock are singed (one in front and the other behind) but the Hostiles manage to grab a few of the torches as they beat a retreat.
Back at the Hostiles cave, the men show the fire to the women-- now they have fire as well. Ock recites the story of how they stole the fire from Lar and Atouk and the Misfits. As the tribe begins to carry on its business, Tonda looks around, putting his hands on his hips. Tonda shows he is not as dumb as he is strong, after all-- he calls Ock over and straightens out his spine. He does the same for Ock's mate, and then Lana. The Hostiles are impressed at how they are suddenly standing taller and straighter. Finally it is Tonda's turn, but his arched spine is so stiff, that it takes all of the tribe's men in a huge dogpile to straighten his spine out. Tonda sighs with relief and pleasure as his spine finally aligns. He stands up, now much taller and straighter, and smiles proudly as the tribe cheers.
The Misfits note that their fire is gone. Atouk realizes Tonda and the Hostiles have it, and organizes a party to steal it back. They sneak into the Hostiles cave, discovering it empty, and take back their fire, putting out the remnants.
The entire Hostiles tribe, women as well as men, are gathering fish from a river (the men are using the women as fish-grabbers). A crab grabs Tonda's nose in its claw, distracting him and causing him to let go of Lana, who suddenly gets swept off in the river's current. Atouk and Lar, who had lagged behind from their tribe's recovering their fire, spot Lana being swept downstream. They manage to rescue her, but then Lar falls in and is swept off, disappearing. Atouk cannot save Lar and turns back to Lana. Despite her haughty nature, Lana seems temporarily saddened at the apparent loss of Lar, understanding he helped save her life.
Atouk brings Lana back to the Misfit camp. Much of the tribe begins to dote on the new arrival-- except of course for Tala, who recognizes Lana and glares jealously at her from a little further back, where Gog, understanding that Tonda's mate is in the camp, stands by surprised. The tribe asks where Lar is, and Atouk tries to explain that he is gone. But the tribe's men refuse to accept this, and finally Atouk organizes the men to take up torches and form a search party. Lana plays up to Atouk, acting like she now loves him, and he is reluctant to leave her side, but the tribesmen make him come along to search for Lar. Tala glares at Lana and then her face lights up with an idea, and she rushes off.
The Misfits find Lar frozen alive in ice in a distant ice cave. They melt the ice with the fire and thaw Lar out-- and find out he was fleeing an abominable snowman, also frozen in the ice. The fire thaws out the abominable snowman too and it chases them around the cave in another amusing chase sequence before they give him the slip.
Meanwhile, Tala has gone to the Hostiles cave where she sees Tonda in the process of choosing a new mate. Tala interrupts the process and explains to Tonda that Lana is alive. She urges him to follow her, gesturing that she'll lead him to Lana. Tonda is disbelieving at first but then orders the tribesmen to come along and follow Tala.
However, Tala finds to her horror and dismay that her plan to get rid of Lana tragically backfires as the Hostiles begin to carry off the Misfits tribeswomen. She jumps on Ock's back, but he catches her easily and begins to carry her off as well.
The Misfits men arrive back at camp, finding Gog alone. Gog had smelled Tonda and recognized his scent, and tells Atouk and Lar that Tonda is responsible for seizing all the Misfits women. Incensed, Lar grabs up a stick and starts to rush off, but Atouk stops him. suddenly he picks up a stick and a bone, showing them to the tribe.
We see the Misfits quickly fashioning weapons from sticks and bones, and armor from bone pieces. Atouk develops a sling and a rudimentary catapult. He also sets a line of melons filled with the sleep-inducing berries, and calls out to the quadrupedal dinosaur, who begins munching on them. The Misfits having finished crafting their armor and weapons, march off to war.
They use the catapult to launch a nervous Ta up to the top of the Hostiles cave. There he quickly uses two rocks to ignite a tumbleweed and drop it into the cave. The smell and smoke forces the Hostiles men out, where they find Atouk challenging Tonda. Quickly the Misifits rush out and the men of both tribes begin battle. Atouk jabs Tonda in the stomach with his weapon but then suddenly rushes off. The battle goes back and forth until the armor and weapons made by the Misfits begins to win out. Suddenly Atouk's shout is heard and everyone turns to see him riding atop the quadrupedal dinosaur. Tonda sends several Hostiles to rush at Atouk, but using his weapon and maneuvering the dinosaur mount to use its spiked tail, he defeats them all. Finally Tonda rushes forward himself. He manages to move around the dinosaur and grab Atouk, throwing him to the ground. He starts to soccer-ball kick Atouk up and down the battlefield. Lana suddenly rushes forward and adds a kick of her own, but Tala then comes up behind Lana and taps her on the shoulder, and then bashes her across the face.
Atouk manages to grab a bone and whack Tonda across the forehead. He snatches up a large bone carapace used as a shield by one of the Misfits and bashes Tonda with it. Finally he uses the sling, throwing a stone that hits Tonda in the head and knocks him down. Atouk jumps onto Tonda's chest, jumping up and down and stomping on him until everyone sees that Tonda is subdued and barely conscious. Atouk declares Tonda unseated and himself the victor, and everyone shouts his name, even the Hostiles acknowledging Atouk as the winner and the new leader of the now-unified tribe.
Lana wastes no time sauntering suggestively up to Atouk, and he smiles as he takes her hand. Lana makes a smug face at Tala as Atouk leads Lana off. Tala begins to cry as Atouk leads Lana up the rock face to stand with her in front of the cave's entrance. He picks Lana up as all the tribespeople begin to cheer... except of course for Tala, who cries dejectedly on Gog's shoulder.
But suddenly Atouk turns to one side and drops Lana off the rock face into a pile of dinosaur dung. He brushes his hands off against the other as the tribespeople shake their heads at Lana's appalling fate. Lana squirms and flounders around in the dung, grunting and crying out loudly in dismay. Atouk turns and looks at Tala, who stands with a shocked but elated look. Slowly Atouk makes his way to her, and she smiles. He leads her up to the cave entrance and presents Tala as his mate and the new Alpha female as the tribespeople cheer. Tala elatedly hugs Atouk (sorry folks, no kissing in this movie, it's rated PG...also, they haven't invented kissing yet!), and the tribespeople all enter the cave to resume life... with Atouk and Tala as the Alpha pair, and Tonda and Lana relegated down to the Omega pair. The screen goes black and a caption reads, "And they lived happily ever after," before the credits begin to roll.