When champion sharpshooter Frank Butler (Howard Keel), his personal manager Charlie Davenport (Keenan Wynn) and Buffalo Bill Cody's Wild West troupe of headliners At the winter quarters of the Ringling Bros.-Barnum & Bailey Circus in Sarasota, Florida, dedicated, no-nonsense manager Brad Braden (Charlton Heston) meets with Ringling executives, led by John Ringling North, and is told that the circus cannot afford to play small towns during the upcoming season. After Brad reveals that he has just signed popular European aerialist The Great Sebastian (Cornel Wilde), however, the executives, anticipating brisk ticket sales, agree to a full tour.

Although most of the performers are grateful for the extra work, Holly (Betty Hutton), the circus' ambitious trapeze star, is devastated, as she knows that Sebastian will demand the center ring. Despite being in love with Brad, Holly tearfully reminds him that he had already promised her the center ring and accuses him of betraying her. That night, still brooding, Holly discusses her feelings with kind-hearted clown Buttons (James Stewart), who helps her to see that Brad was merely putting the best interests of the circus first. Holly also admits she is confused about her love for Brad, whose obsession with the circus overshadows their romance. Buttons comments that people often kill the thing they love most, then states that he "loved once" and will never love again.

Later, as the circus train is about to depart on its tour, Sebastian zooms up in his sports car, trailed by several ticket-wielding policemen. The handsome Sebastian, who enjoys a reputation as a trouble-making womanizer, is greeted warmly by his former lover, the "iron-jawed" Phyllis (Dorothy Lamour), but Angel (Gloria Grahame), the Elephant Girl, another ex-paramour, scorns him. Sebastian is immediately attracted to Holly and offers to give her the center ring, but Brad refuses to make the switch. Frustrated, Holly issues Sebastian a challenge, bragging that she will outdo him in the air until Brad is forced to award her the center ring. Holly's dare generates great publicity, and audiences delight in watching the young woman match Sebastian stunt for stunt. As the stunts become more difficult, however, Buttons and Brad grow concerned, and Brad finally orders Holly to stop the "dog fight." Holly, however, enjoys the thrill of the duel and insists on continuing.

When the worldly Angel warns Holly not to sacrifice Brad for Sebastian, Holly retorts that Brad only loves the circus. Later, during a performance, Buttons, who never removes his clown makeup, spots his mother in the audience and surreptitiously speaks with her. The old woman warns Buttons that the "men" are still looking for him, but he assures her that he is safe as long as he remains in costume. Brad then upsets Holly when he yanks her down in mid-trick and the audience laughs at her. Furious, Holly allows Sebastian to romance her after the show until Angel directs one of her elephants to scoop her up with his tusks and deliver her to Brad. After Brad explains that he stopped the stunt because the rope she was using was about to break, Holly forgives him and agrees to end the duel.

Meanwhile Sebastian, who senses he is losing Holly, announces he is performing a new, dangerous stunt, and when Holly teases him about the safety net that Brad has ordered for the center ring, Sebastian disengages the net before going on. Unprepared for the stunt, Sebastian misses his mark and falls to the ground, stunning the crowd. After the broken Sebastian is carried off on a stretcher, Brad tells Holly she is "center ring" now, and Holly cries with guilt. Sometime later, a recuperated Sebastian strolls into the circus camp and tells Holly and Brad that he has been hired by a rival circus, whose women, he hears, are especially beautiful. Holly is hurt by Sebastian's apparent fickleness, but Brad senses that Sebastian is lying and grabs his coat off his arm, exposing Sebastian's crippled right hand.

Surmising that Sebastian returned only because he is in love with her, Holly declares her love and insists on taking care of him. One night, after Angel makes a final attempt at discouraging her about Sebastian, Holly notices a discarded magazine article about a doctor "who killed the thing he loved." Holly shows the item to Buttons, pointing out the phrase Buttons had used with her, but Buttons shrugs it off. However, Brad is approached by a detective who is on the trail of a doctor wanted for the mercy killing of his terminally ill wife. Brad claims not to recognize the photograph the detective shows him but gives him permission to fingerprint some of the performers on the train. Buttons, meanwhile, discovers that Sebastian, who now works as a balloon seller, has some feeling in his bad hand, a sign that the damage is not irreparable, and later on the train, reports the good news to Brad.

Although Brad and Buttons have never discussed Buttons' past, Brad has deduced that Buttons is the fugitive doctor and warns him about the detective. Brad then deliberately provokes Sebastian into taking a swing at him, and Sebastian realizes that he has sensation in his damaged hand. Overjoyed, Sebastian races to see Holly and, believing that he will be able to "fly" again, proposes. Just then, however, their train cart collides with a stalled train ahead, and passengers and animals are sent flying. Brad is pinned under some rubble and the circus doctor is knocked out. Desperate to save the bleeding Brad, Holly, who has also figured out Buttons' identity, asks the clown to tend to his wound, and although he knows that by doing so, he will expose himself to the detective, Buttons agrees.

When Buttons declares that a transfusion is needed, Sebastian, who has the same rare blood type as Brad, volunteers. The transfusion is a success, and Brad is saved. Brad then orders that the show go on, despite the destruction of the big top, and Holly suggests they hold the circus in the nearby field. Buoyed by her rekindled love for Brad, Holly directs the others in setting up the impromptu circus and leads a colorful parade into Cedar City, their next scheduled stop. Back at the train wreck, Henderson offers to buy Brad's "washed-up" venture and is stunned when a huge crowd comes marching up, eager to buy tickets. After Buttons gives himself up to the detective, Brad declares his love to Holly, and Sebastian proposes to Angel. Under the bright blue sky, the Greatest Show on Earth then goes on.

Betty had never been to a circus prior to her work on "The Greatest Show On Earth". Her family was too poor to afford tickets when she was a kid and she'd been too busy after finding success to go.

Betty campaigned greatly for the lead role and early on had anonymously sent director Cecil B. DeMille a $1,000 floral piece--a complete circus made of flowers with a replica of her as a trapeze artist in the middle.

Cecil B. DeMille was always demanding of his actors and actresses. He insisted that everyone truly learn to perform the circus stunts they were supposed to be performing. This meant that Betty really learned the trapeze.

Antoinette Concello of the Flying Concellos was Betty's coach for this film.

DeMille had a tradition of handing out specially minted half-dollars when he was pleased with an actor's work. Betty received four.

DeMille was quoted as saying, "Betty is one of the three greatest feminine troupers I have directed in four decades of motion pictures."

While filming this movie, DeMille presented Betty with the Photoplay Award for favorite actress of 1950 for "Annie Get Your Gun". The presentation was filmed and shown on a newsreel.

During one scene Cornel Wilde is hanging from the trapeze by his knees. He catches Betty then pulls her up and kisses her. This shot took several takes and during one of the early takes Wilde tore the ligaments in his shoulder. He managed to make it through two more takes, then had to stop. He was unable to use his arm for several days so De Mille shot scenes where he was not needed.

Special effects produced a green halo around Betty and Gloria Grahame in the Grand Parade scene, so a shot was added of green floodlights turning on above them.

Lucille Ball was Cecil B. DeMille's first choice for "Angel", but she became pregnant and had to be replaced by Gloria Grahame.

Bob Hope, Bing Crosby, Diana Lynn, Merrill Reese, and Mona Freeman made cameos as circus spectators.