In 2 million B.C, an asteroid collides with the prehistoric Earth, causing the extinction of planet’s dinosaurs, but sparing a tribe of cavemen living near the impact site. Finding a roughly spherical chunk of the asteroid that is too hot to touch, the cavemen begin to kick it around and invent the game of football.
Centuries later during the Stone Age, in the year 1 Million B.C., the impact site has become a 100,000-acre valley. Living in the valley is a young caveman named Dug along with the chief Bobnar, many other cavemen such as Asbo, Gravelle, Treebor, Magma, Barry, Grubup, Thongo, and Eemak, and his pet boar Hognob. One day, Dug suggests to Bobnar that they should try hunting woolly mammoths instead of rabbits as they always do, but Bobnar brushes him off, believing the tribe could not catch mammoths.
Later that night, a mysterious army of war mammoths led by Lord Nooth (the leader of the Bronze Age) in the year 3000 B.C. drives the tribe out of the valley and into the surrounding volcanic badlands, proclaiming that the Stone Age has ended and that their time has begun. Dug tries to attack the army, but falls into a cart and is unknowingly taken to Nooth’s 10,000-acre Bronze Age city.
While trying to evade the guards and escape, he ends up mistaken for a football player and led onto the pitch before a full stadium crowd. He challenges Nooth’s elite local team to a match with the valley at stake and promises that the tribe will work in Nooth’s mines forever if they lose. Nooth dismisses the proposal at first, but changes his mind once he realises that he can profit from the match.
Nooth later receives a message bird from Queen Oofeefa, having got word that Nooth’s football team will challenge the cavemen. Nooth believes his team will win but Oofeefa warns him to not underestimate Dug’s team.
Dug discovers that although his ancestors played football, the other members of his tribe are too dim to understand it. They get chased by a 36-foot tall duck which ends up destroying their only ball. Later that night, Dug and Hognob sneak into the Bronze Age city to steal more balls but are found by a 25-year old resident named Goona.
Resentful over the team’s exclusion of women, she helps them steal some balls and agrees to coach the cavemen. Goona points out that the players on Nooth’s team are talented but too egotistical to work together effectively. The cavemen improve in skill and teamwork under her coaching.
Nooth learns from his men working in the mines that the cavemen’s ancestors invented football from cave paintings. He receives the Message Bird from Oofeefa again and she has also learned the cavemen’s ancestors invented football as well as the fact that they have been training every day and improving. Oofeefa concludes the message by telling Nooth that he will work in the mines if he disappoints her.
Two of his men working in the mine come with copies of more cave paintings which give Nooth an idea. To demoralise Dug, Nooth has him brought to the mines and shows him cave paintings made by his tribe’s ancestors who, although they had invented the game and taught other tribes to play it, proved so inept at football than other tribes that they never won a single match and eventually gave up the sport.
Nooth then offers Dug a deal which he later agrees to. On the day of the match with Oofeefa in attendance, Dug announces his forfeiture as part of the deal which spares the rest of the tribe and agrees to take their place in the mines alone.
However, his reinvigorated teammates arrive on the now tamed giant duck and persuade him to break the deal and play the match. They are down 3–1 at half-time, but rally in the second half to tie the score. Nooth incapacitates the referee and takes his place, making biased calls in favour of the local team that leads to Bobnar, who is the cavemen’s goalkeeper, being knocked out.
Hognob takes his place and blocks a penalty kick, and Dug scores using a bicycle kick to win the match for the cavemen, 4–3. The cavemen win their valley back with the respect of Oofeefa, the local team, and the crowd. Nooth tries to escape and steal the crowd’s admission money, but Dug and Goona stop him with help from the giant duck. Nooth is arrested for his crimes and everyone gets their money back.
Goona and Nooth’s elite local team join Dug’s tribe for a hunt, but they are frightened off by a rabbit pretending to be a woolly mammoth. What would happen next?