It’s 1979 and the opening night for “The Barracuda," New York City’s first floating casino! Chad is looking for some ladies ("Hot Stuff") and decides to join his best friend, Scott, as a waiter on the casino boat.
Meanwhile, on the pier, Professor Ted Scheider is collecting water samples to determine whether the new pier has been drilled directly into a dangerous fault line. Marianne, a freelance reporter, has heard that Tony Del Vecchio, the owner of the Barracuda, ran out of money and had to cut corners. He asks her to join him onboard for some drinks, and she agrees, hoping to get a scoop.
While the guests wait in line, a dour Sister Mary Downy appears, collecting money for the orphans fund and warning people that gambling is a sin ("The Lord's Prayer"). She meets Shirley and Maury Summers, who are celebrating Maury’s retirement. Despite the Sister’s protestations, Shirley buys her a ticket, and she enters the casino.
Levora Verona, a faded disco star, runs onto the pier with her precious dog, Baby. She successfully avoids a cab driver whom she can’t afford to pay and gets onboard, hoping that she’ll win back her fortune ("Mahagony").
Inside her casino dressing room, a sexy lounge singer, Jackie, excitedly tells her children, eleven-year-old twins, Ben and Lisa, that Tony told her, “If tonight goes perfectly, he will ask me to marry him... possibly!” Tony enters just as a tremor shakes the room, causing Ben’s Lite-Brite toy to fall to the ground. Ben cries as he notices that "the Lite-Brite pieces are lost in the shag carpeting!" Tony explains that the tremor was probably caused by nearby construction and tells Ben that he should never, ever cry. Jackie then goes onstage to wow the opening night crowd ("Saturday Night"). During her number, Professor Scheider warns Tony that the vibrations from everyone dancing is going to trigger an earthquake, and Tony angrily throws him off of the ship.
Meanwhile, Sister Mary is practically salivating over a “Hawaii 5-0-”themed slot machine but, when noticed by Shirley, adamantly denies having a gambling addiction.
Marianne tries to interview Tony, but he’s only interested in one thing, ("Do You Wanna Make Love"). She accuses him of having a trap door installed at the bottom of the ship to dump illegal contraband and produces proof that he skipped safety precautions. Tony panics but is able to sneak away when Marianne freezes at the sight of Chad, who has offered them drinks. Shaken, she apologizes to Chad for leaving him at the altar years ago. He tells her that it’s “fine" and then retreats to the bathroom to express his true feelings ("Without You"). Hurt by Chad’s indifference, Marianne drinks an entire bottle of wine and then runs into Lisa, trying to convince her (and herself) that being an independent woman is the most important thing in the world ("I Am Woman / That's the Way I’ve Always Heard It Should Be").
Jackie is in her dressing room when Professor Scheider runs in to hide. He explains that an earthquake is coming and enlists Ben and Lisa to run to tell the captain to ready the lifeboats. Tony suddenly enters, and Professor Scheider quickly throws on a bird mask. Jackie covers by telling Tony that Professor Scheider is a performer whom she hired and drags him onstage ("Mockingbird").
Shirley and Maury run into a depressed Marianne, who is surprised to find out that they’re married because they “seem so happy.” They admit that marriage is hard but tell her that they still love each other dearly ("Still the One"). While Professor Scheider hides out with Jackie, he reveals that his first wife, Dr. Wo-Ching Lee, died in a volcanic explosion and that he feels responsible. Since then, he’s become a disaster expert and tells Jackie that one of the main rules is not to try to save individual lives. When Jackie leaves to perform, we see three private moments: Chad sings about Marianne, Marianne sings about Chad and Professor Schedier sings about Wo ("Feelings").
In the casino, Sister Mary’s out-of-tune guitar upsets Levora’s dog. Levora is extremely insulted because Sister doesn’t know that any of her hit songs including “Makin’ Sweet Sweet Love” and “Put It Where I Want It.” Levora leaves in a huff, unknowingly dropping a quarter. Sister Mary knows that she should give it to the orphans but imagines gambling it and winning them a pile of money. Nonetheless, she tries to walk away from the slot machine... but is unable to do so ("Never Can Say Goodbye").
Sister ends up gambling away all of the orphans’ fund money that she’s raised. She bumps into Shirley on the elevator and they ride in uncomfortable silence, interrupted only by elevator music ("Feels So Good"). Suddenly, Shirley reveals that she has a fatal disease and explains that she’ll soon show the final warning signs, like uncontrollable winking and inappropriate verbal outbursts. She hasn’t told Maury because she wants whatever time they have left to “only be happy." Shirley shyly suggests that she might soon end it all because she doesn’t want Maury to have to watch her suffer. The Sister tells her unequivocally that taking one’s life is a sin, and an eternity in hell will await her if she does it.
Ben and Lisa run into Chad and tell him that an earthquake is coming, and he immediately runs off to warn Marianne.
In the casino, a wealthy lady approaches Levora and offers to buy her precious dog, Baby, for an extraordinary sum. Levora can’t bear to part with the only thing that she’s ever loved, yet she has only one quarter left to gamble and gain back her fortune. So, she knocks on the wood-paneled slot machine for luck and talks to Baby ("Knock on Wood"). Soon, various patrons are knocking along with her, on the machines, the floor… everything.
While this is happening, Marianne confronts Tony about the safety precautions and runs away as he tries to throw her off the boat. Chad sees Marianne hiding and tells her to get off of the boat, but she misunderstands, thinking that Tony has sent him. Professor Scheider has heard the knocking and runs to the casino to warn everyone that they’re going to trigger an earthquake. They stop and start to tiptoe off the boat, but Levora has put in her quarter... and wins ("Hawaii 5-0"). All of the coins pouring out of the machine hit the floor... and trigger an enormous earthquake!
Act Two opens ("All Right Now") to reveal massive destruction in the casino... as well as the “Hawaii 5-0” slot machine lever that is impaled in Scott’s stomach. Chad arrives and tries to get Scott to safety ("You’re My Best Friend"), but Scott dies.
Shirley, Maury, Jackie and Lisa are all in Jackie’s dressing room, but Jackie soon leaves to find Ben, who’s missing.
In a deserted hallway, Tony explains to his right-hand man that the earthquake actually solved his problems. It will be considered a force majeure, and all of his debt will be paid! And he’s positive that he’ll end the night being seen as a hero.
Chad is on the deck to give Scott a burial at sea and runs into the wealthy man whose wife was killed by a falling ice sculpture ("Three Times a Lady").
While waiting in Jackie’s dressing room, Shirley starts showing signs of her impending death (inappropriate verbal outbursts), and Lisa discovers Ben, who is passed out because of his diabetes. Lisa volunteers to carry Ben to the infirmary to get insulin, and Maury and Shirley follow.
On the way, they meet the sister who is praying for death because she thinks that the earthquake was her punishment. She’s certain that God wants her in hell, but Shirley convinces her that God actually wants her to help other people, so they all set off for the insulin.
Lisa spies the infirmary, and they run to it ("Ben"). Ben is revived in the nick of time. Everyone in the casino is panicking because the earthquake has caused the boat to detach from the pier, but Tony is calm and directs people to the lifeboats. However, Professor Scheider warns that a tidal wave is coming and quickly ushers them into a hallway. Upon learning this, Marianne runs out onto the deck and gets Chad off of his lifeboat ("Baby Hold on to Me") and back onto the ship. The tidal wave hits, and the ship turns upside down. Professor Scheider assures everyone that the safest thing to do is stay put, but Tony boasts that he’ll save everyone through an exit ladder in the kitchen. Professor Scheider objects, but Tony assures them that the kitchen has fire doors and is therefore safe, so half of the survivors, including Jackie, Ben and Lisa, leave with him. The others choose to wait it out with Professor Scheider ("25 or 6 to 4"). Soon, there is an enormous explosion from the kitchen, and only Tony reappears. The Professor realizes that Tony lied about the fire doors, and they all express their anger at him as Chad also shares his anger with Marianne for not showing up to their wedding ("Sky High").
Jackie and her children survived the blast but are hanging from a wall. She hopes that Tony will come and rescue them, but the kids know better ("When Will I Be Loved").
Professor Scheider decides to buck his own rules, braves the fire and arrives to rescue them with a daring routine on a high beam ("Nadia's Theme"). On their way back to the casino, they hear Tony, who’s trapped in a flooded room with sharks ("Don't Cry out Loud").
The explosion has separated Levora from her dog, Baby. The sister appears and helps Levora find him by strumming her guitar until he barks ("Come to Me").
Marianne admits to Chad that she did love him but was scared that their marriage would make them as miserable as her parents. They find themselves in a room that’s flooding and filled with man-eating piranhas. The only way to escape is through a water-tight door, but it’s locked. Perched together atop a pile of chairs, Chad admits that he regrets not calling her for years. ("I Really Want to See You Tonight"). Suddenly, they hear sounds above them and start banging Morse code on the ceiling, screaming for help ("Knock Three Times").
In the casino, Jackie and her kids reappear, and Tony thanks Professor Scheider for rescuing him. He tells little Ben that it’s good to cry every day, to which Ben responds, "I already do." Tony tries to reconnect with Jackie, but she decides to end their relationship ("I Will Survive").
Shirley hears Chad and Marianne’s Morse code, and Professor Scheider informs her that he has instructions on how to unlock the door in his Disaster Handbook. Shirley tells him that she was a tap dance champion and will tap out the instructions. The Sister quietly asks Shirley if it’s wise to exert herself because it might lead to death, but Shirley tells her that she might as well “go out helping people,” and the Sister blesses her decision. Shirley taps out the instructions ("A Fifth of Beethoven"), Chad and Marianne are saved and pledge their love for each other ("Reunited") and Shirley dies in the arms of Maury.
A swarm of rats attack, so everyone rushes to safety in Tony’s private office, where Tony apologizes for everything and finally admits that there is a trap door on the bottom of the ship. Ben points out that the ship is upside down and, therefore, the trap door is on the ceiling!
They all escape onto the top of the ship as the sun rises ("Daybreak"). A helicopter appears and drops harnesses. As survivors buckle themselves in, Professor Scheider reveals his feelings for Jackie ("Hooked on a Feeling"), and they all begin flying off to safety. Because the pier was destroyed, the helicopter is taking them to New Jersey. When Levora hears where they’re heading, she exclaims, "New Jersey? This is the worst disaster yet!"