The California Atoms are a professional football team in Los Angeles, owned by Hank Cooper (Ed Asner) and coached by the inept Coach Venner (Don Knotts). They are by far the worst team in the league and have not won a single game in years. Their cheerleaders are fat and ugly, and even the marching band is awful. Desperate to draw fans in somehow, Cooper wants a great half time show. His secretary, Debbie (Louise 'Liberty' Williams), sees a story in her parents Yugoslavian newspaper about Gus, a mule that can kick a soccer ball through the goal from anywhere on the field.
Debbie flies to Yugoslavia and meet with Andy Petrovic (Gary Grimes), the young man who owns the mule, Gus. Andy's older brother is a local soccer star, and their parents idolize the brother and are very critical of Andy since Andy is terrible at soccer. Gus and Andy are hired to come to L.A.
Meanwhile, Cooper is a compulsive gambler who owes a lot of money to two mobster bookmakers Charles Gwynn (Harold Gould) and Cal Wilson (Dick Van Patten). When Cooper tells them that he cannot pay them for he is broke, they give him a last chance bet: if the Atoms win the Super Bowl, he owes them nothing, but if they do not win the Super Bowl, the gamblers get the team from him... and implies that they might do him physical harm or worse, kill him.
The Atoms are losing their first game, but Gus is a hit at halftime when he shows off his skills by kicking a football the entire lenght of the field. In the 2nd half, Gus is put into the game to kick a field goal. Andy tells the other players to throw the ball to him and Gus will kick a touchdown. (An obvious reference to Garo Yepremian of the 1972 Super Bowl Dolphins who notoriously infuriated Larry Csonka by running off the field yelling "I kick touchdown"). The other team protests, and the announcers Pepper (Bob Crane) and Johnny Unitas (playing himself) are aghast. However, nowhere in the rule book does it say that players must be human, and the refs allow Gus to kick. With Gus making a field goal on each possession from anywhere in the field, the Atoms win.
The Atoms go on to win their next few games thanks to Gus getting field goals every time, and are in first place in their divison. However, one catch is that Gus will only kick when Andy holds the ball and shouts the command: "oyage". Debbie has been assigned by her boss to watch over Andy and Gus, since she can speak his Serbian language, and there is an obvious romance budding between them. Gus is an intelligent mule, and it is clear he is trying to push them together.
Meanwhile, the gamblers Gwynn and Wilson are getting upset about the team's success. Not wanting the Atoms to win, they hire two incompetent criminals/con artists, Crankcase (Tim Conway) and Spinner (Tom Bosley), to stop Gus from playing in the games. For the next game, Crankcase shows up impersonating a replacement driver with a horse trailer. He gets Andy and Gus lost in the desert so they miss the game. The Atoms lose. Cooper knows the gamblers were behind this, so he hires security guards for Gus and Andy.
After a few more victories, Spinner dresses up as a police official and goes to "review the security" around Gus. While no one is looking, he spikes Gus's water pail with alcohol. A drunken Gus stumbles around the field and misses his kicks, and even falls on and crushes the football.
Still with only two loses, the Atoms make the playoffs. Andy is a celebrity and is on a TV show. His parents appear on the show from Yugoslavia. They are unimpressed since he just holds the ball for Gus to kick. The father keeps saying that his older brother, a pro soccer star in their home country, is the real star.
Before the playoff game, Spinner and Crankcase call Andy and tell him that Debbie has been in a car accident and is seriously injured. Andy runs to the hospital where Spinner is impersonating a doctor and he locks Andy up in an empty room. In the meantime, the Atoms are losing, with Gus refusing to kick for anyone but Andy. Andy manages to escape from the hospital and, wearing nothing but his hospital gown, attempts to reach the stadium before the game is over. However, when Debbie dresses up in Andy's uniform and asks Gus to kick for Andy, Gus does kick. The Atoms win with Debbie as holder, although everyone in the stands thinks it's Andy. Andy arrives when the game is over and is very upset since it is like his father said, that he is nothing, that anyone can hold the ball for Gus to kick.
When the Atoms reach the Super Bowl, Spinner and Crankcase steal Gus when Spinner impersonates a bus driver being stuck in the middle of a country road, and he distracts Andy and Cooper long enough to help him fix his bus while Crankcase steals Gus from his trailer and replaces him with another mule. Spinner and Crankcase then check into a local motel and lock up Gus with them.
On the day of the big final game, Crankcase and Spinner turn on the TV set in their motel room to gloat as they plan to watch the Atoms lose. When Gus sees the Super Bowl starting on TV, he goes wild and kicks the TV set to pieces. In a bizzerk rage, Gus then kicks down the door to escape from his captors. At the Super Bowl, when the mule refuses to kick the football, Andy knows that it is not Gus, so he and Cooper leave the game and take off in a helicopter to search for Gus.
In a long and comic chase sequence, Crankcase and Spinner chase Gus into a local supermarket where they attempt to recapure the intelligent mule, only he gives both bumbling thugs the slip. When Spinner tries to lasso Gus, the mule drags Spinner along a shelf and into a lobster tank. Crankcase is disposed of when he jumps on Gus' back, and the mule then runs down a supermarket row and throws Crackcase off his back and into a wedding cake display. Running from the supermarket, Gus is spotted from the air by Andy and Cooper and they land in the supermarket parking lot to collect him. They airlift Gus to the Super Bowl by half-time.
With Gus back in the game, the Atoms come back hard, as Gus makes several field goals kicks. With seconds left on the clock, the Atoms are down 12 to 14, with the ball on their own 5 yard line. It will come down to this last field goal attempt. But on this last kick, Gus slips in the mud and misses the football. There is a scramble for the ball, and Gus kicks it to Andy standing off on the side. Andy takes the football and runs 95 yards for the touchdown and wins the game for the Atoms.
After the game, Andy is hailed as the hero... thus leaving Gus in the background for the first time. While Coach Venner congagulates Andy, Cooper tells Gus: "it's okay, everyone misses once". After Cooper walks off to celebrate with Andy and the rest of the team, Debbie winks at Gus and says: "I know you did that on purpose". Gus winks back at her.