Note: There is no definitive version of the libretto of Show Boat, although the basic plot has always remained the same; minor revisions have been made by the creators, and subsequent producers and directors over the years.

Act I
In 1887, the show boat Cotton Blossom arrives at the river dock in Natchez, Mississippi. Its owner Cap’n Andy Hawks introduces his actors to the crowd on the levee. A fist fight breaks out between Steve Baker, the leading man of the troupe, and Pete, a rough engineer who had been making passes at Steve’s wife, the leading lady Julie La Verne. Steve knocks Pete down, and Pete swears revenge, suggesting he knows a dark secret about Julie. Cap’n Andy pretends to the shocked crowd that the fight was a preview of one of the melodramas to be performed. The troupe exits with the showboat band.

A handsome riverboat gambler, Gaylord Ravenal, appears on the levee and is taken with eighteen-year-old Magnolia (“Nolie”) Hawks, an aspiring performer and the daughter of Cap’n Andy and his wife Parthy Ann. Magnolia is likewise smitten with Ravenal (“Make Believe”). She seeks advice from Joe, a black dock worker aboard the boat. He replies that there are “lots like [Ravenal] on the river” and, as Magnolia excitedly goes inside the boat to tell her friend Julie about the handsome stranger, Joe mutters that she ought to ask the river for advice. He and the other dock workers reflect on the wisdom and indifference of “Ol’ Man River”, who, no matter what, “jes’ keeps rollin’ along”.

Magnolia finds Julie inside and announces that she’s in love. Julie cautions her that this stranger could be just a “no-account river fellow.” Magnolia innocently retorts that if she found out he was “no-account,” she’d stop loving him. Julie warns her that it’s not that easy to stop loving someone, explaining that she’ll always love Steve (“Can’t Help Lovin’ Dat Man”). Queenie walks in and asks why Julie knows that song; Queenie says she has only heard “colored folks” sing that song. Magnolia says that Julie sings it all the time, and when Queenie asks if she can sing the entire song, Julie obliges.

During the rehearsal for that evening, Julie and Steve learn that the town sheriff is coming to arrest them. Steve takes out a large pocket knife and makes a cut on the back of her hand, sucking the blood and swallowing it. Pete returns with the sheriff, who insists that the show not go on, because Julie is a mulatto woman married to a white man, and local laws prohibit miscegenation. Julie admits that she is a mulatto, or mixed race. Steve, because he swallowed Julie’s blood (and therefore has at least “one drop of black blood”), claims he is also mulatto. The troupe backs him up, boosted by the ship’s pilot Windy McClain, a longtime friend of the sheriff. The sheriff lets the couple go, but they prepare to leave town because blacks were prohibited at the time from acting onstage. Cap’n Andy fires Pete for his actions. Gaylord Ravenal returns and asks for passage on the boat. Andy hires him as the new leading man, and assigns his daughter Magnolia as the new leading lady, over her mother’s objections. Julie leaves with Steve.

Weeks later, Magnolia and Ravenal have been a hit with the crowds and have fallen in love. He proposes to Magnolia, and she accepts. (“You Are Love”) The two become engaged and make plans to marry the next day while Parthy, who disapproves of him, is out of town. Parthy has discovered that Ravenal once killed a man. She arrives with the Sheriff at the wedding festivities, but the Sheriff says that Ravenal was acquitted. Cap’n Andy calls Parthy “narrow-minded” and defends Ravenal by announcing that he also killed a man. Parthy faints, and the wedding party proceeds with the ceremony.

Act II
Six years have passed, and it is 1893. Gaylord and Magnolia have moved to Chicago, where they make a precarious living from Gaylord’s gambling. By 1903, they have a daughter, Kim, and after years of varying income, they are broke and rent a room in a boarding house. Depressed over his inability to support his family, Gaylord leaves Magnolia. Frank and Ellie, two actors on the boat looking for a place to live, discover that Magnolia is living in the rooms they want to rent. The old friends seek a singing job for Magnolia at the Trocadero, the club where they are doing a New Year’s show. Julie is working there, but has fallen into drinking after having been abandoned by Steve. At a rehearsal. she tries out the new song Bill, and while singing it, she is obviously thinking of her husband and performs the song with great emotion. From her dressing room, she hears Magnolia singing “Can’t Help Lovin’ Dat Man” for her audition, the song Julie taught her years ago. Julie secretly leaves her role so that Magnolia can fill it without learning of her sacrifice.

On New Year’s Eve, Andy and Parthy go to Chicago for a surprise visit to Magnolia. He goes to the Trocadero without his wife, and sees Magnolia overcome with emotion and nearly booed off stage. Andy rallies the crowd by starting a sing-along of the standard, “After the Ball”. Magnolia becomes a great musical star.

It is now 1927. An aged Joe on the Cotton Blossom sings a reprise of “Ol’ Man River”. Cap’n Andy has a chance meeting with Ravenal and arranges a reunion with Magnolia. Andy knows she is retiring and returning to the Cotton Blossom with Kim, who has become a Broadway star. Ravenal sings a reprise of “You Are Love” to the offstage Magnolia. Although uncertain about asking her to take him back, Magnolia, who has never stopped loving him, greets him warmly and does. As the happy couple walks up the boat’s gangplank, Joe and the cast sing the last verse of “Ol’ Man River”.

Note: The 1951 MGM film changed many aspects of the story. It brought Ravenal and Magnolia back together only a few years after they separated, rather than 23. Gaylord has a chance meeting with Julie, and learns that he has a daughter. Gaylord returns to find the child Kim playing. Magnolia sees them and takes him back, and the family returns to the show boat. Joe and the chorus start singing “Ol’ Man River” as the scenes unfold, then the paddlewheel starts turning in tempo with the music, as the ship heads down river. Julie is shown, viewing from a distance. She had followed him and watched the scene from the shadows.