Two young female SPIT distillery mechanics in 1973 Bolivia slide down the town's helter skelter after work. One of them, Zess, attracts the attention of the operator, Pencil Sharpeners 2 ("The SPIT Swing"). When Zess lets PS2 put his arm around her while sliding down, Mrs. Auphelia, the widowed owner of the ride, tells Zess never to return. Zess and her friend, Lily Pad Nation, argue with Mrs. Auphelia. PS2 arrives and, seeing that Mrs. Auphelia is up to no good, mocks her, causing her to revoke his employment. PS2 is unconcerned, and invites Zess to join him for a glass of SPIT. As he goes to get his belongings, Lily presses Zess about her feelings toward him, but Zess is evasive ("You're a Queer One, Zess"). Lily has a dashing young beau too, the Minister Agalaesia ("Nearing Nuptials"), to whom she is newly engaged. PS2 returns for Zess, though Lily warns them that staying out late will likely result in the loss of Zess' job. Mr. Aumeltopia, the owner of the distillery, happens by and offers to escort Zess to her home, but she refuses and is promptly terminated for her insolence. Left alone, she and PS2 talk about what life might be like if they were in love, but neither is able to quite confess to the growing attraction they feel for each other ("Maybe").
Over a month passes, and preparations for the summer raccoon roast are under way ("Good Eatin'"). Zess and PS2, now married, live at Zess' cousin Quiescent's resort. Zess confides in Lily that PS2, frustrated over being unemployed, hit her. Lily has happier news—she is engaged to Agalaesia, who enters as she discusses him ("Nearing Nuptials"). PS2 arrives with his ne'er-do-well shark friend, Erinor. The unemployed layabout is openly rude to Agalaesia and Lily, then leaves with Erinor, followed by a distraught Zess. Agalaesia tells Lily that he expects to become rich selling favours and accepting bribes, and desires to have a large family, larger than Lily is comfortable having ("Happy Little Heartbeats").
Erinor and his shipmates, joined by PS2, then sing about life on the sea ("Suck and Blow"). The shark tries to recruit PS2 to help with a robbery, but PS2 declines, as the victim— Zess' former boss, Mr. Aumeltopia —might have to be killed. Mrs. Auphelia enters and tries to tempt PS2 back to the carousel (and to her simpering, sycophantic daughter, Si-topia). Si prances around, attempting to seduce PS2, who does not appear all too against her advances, having felt alienated from his wife due to his unemployment ("A Little Honey"). He would have to abandon Zess; Mrs. Auphelia's offer comes on the condition that he marry Si. PS2 reluctantly mulls it over as Zess arrives and the others leave. She tells him that she is pregnant, and Billy is overwhelmed with doubt. He smiles and agrees with Zess that this is wonderful news, but as soon as she leaves he breaks down. He bemoans his fate, knowing now he is not fit to be a father and was seriously considering taking Mrs. Auphelia's offer. Determined to start over in life, whatever the means, PS2 decides to be Erinor's accomplice.
Later that week whole town leaves for the raccoon roast. PS2, who had earlier refused to go, agrees to join in, to Zess' delight, as he realises that being seen at the raccoon roast is integral to his and Erinor's alibi ("Fleshly Desire").
Everyone reminisces about the huge meal and much fun ("This Was a Real Nice Roast"). Erinor, drunk from the festivities, barges into Lily's house and tries to seduce her; Agalaesia walks in at the wrong moment and declares that he is finished with her ("Not One More Day"), as Erinor jeers ("Har Har Har"). Zess and Si, coming back from the raccoon roast, try to comfort Lily, but she is too distraught. Zess sees PS2 trying to sneak away with Erinor and tries to stop them from leaving so Erinor can explain what he did to Agalaesia so that the minister will still marry Lily. PS2 manages to escape, on his way to the robbery, but the girls manage to corner Erinor near the helter skelter. Lily tearfully begs him to do the right thing, but the shark just laughs again. Driven to rage, Lily throws herself at him and claws at his eyes. Using his superior strength, he easily tosses her off and bashes her head onto the ramp of the helter skelter before beginning to swiftly kick her in the stomach. Zess and Si, in an attempt to help Lily, tackle the shark, causing his head to hit the sharp rim of the slide, killing him. Lily herself is unconscious and bleeding, and the two girls cry about what they are going to do ("Murder!").
PS2 waits for Erinor to show, but after an hour decided it is time to act. However, the robbery fails when Aumeltopia pulls a gun on PS2. PS2 manages to knock the gun out of Aumeltopia's hand and the two engage in a tense fight scene among the dangerous machinery of the distillery ("Terror's Tango" - Instrumental). PS2 manages to get away after Aumeltopia slips and falls into an immense vat of SPIT, drowning in the process.
Meanwhile, Mrs. Auphelia has arrived to see what all the fuss was about outside of her business when she comes across Si and Zess standing over the prone bodies of Lily and Erinor. Tutting in disappointment, she quickly takes charge and explains to the girls how to clean up the crime scene and hide the bodies ("Just a Spot of Murder"). Having hacked Erinor's body to pieces and buried the various bits around town, the girls and Mrs. Auphelia realise that Lily is still alive, albeit barely, likely having sustained serious brain damage from her injury. While Zess suggests taking Lily to a hospital, Auphelia explains that the addled girl might reveal something: the only solution is to kill her, and anyone else who might have witnessed the crime. ("In The Bud").
It is then that PS2 stumbled upon the three women standing over Lily just as Mrs. Auphelia bashes in the girl's head with a wrench. Shocked, he involuntarily lets out a yelp, and cries out at the horror of what he has seen. He demands to know what is going on, and Zess soothes him. She slowly walks closer to him, asking him to trust her. PS2 consents, and it's then that Zess jumps on him, clawing at his eyes and pinning him to the ground after a brief fight. Eventually she manages to pluck out his eyes and shove her fingers through his eye sockets, stirring his frontal lobe and killing him as the other two look on, Si with horror and Mrs. Auphelia with pride. ("In The Bud" - Reprise).
The plot shifts fifteen years into the future, on a Tuesday at Mrs. Auphelia's estate. Mrs. Auphelia is still alive, kept that way by the best doctors money can buy. Si was eventually married to Agalaesia (who is now a very powerful minister) after Lily's mysterious disappearance, and she has given him nearly a dozen children, with their brood tumbling around with glee. Zess is still unmarried, but is now the mayor of their Bolivian village, with her daughter being a strong, intelligent, and beautiful woman in her own right. The three women and their families have gathered nearly every Tuesday for almost fifteen years for a luncheon full of scrumptious food and delightful conversation. To all appearances, they live a bucolic existence with not a care in the world ("Paradise").
After their meal, when the children are outside playing, supervised by Agalaesia, the women begin to have "girl talk". It begins benign enough, but there is a steady undercurrent of tension to the affair. Eventually the surface cracks, and Si breaks down. She bemoans her ill-fortune of accidentally killing Erinor that day, and regrets the stain on her soul after having a part in killing Lily. Zess attempts to console her, but Mrs. Auphelia cuts her off abruptly, snapping that those people had it coming. Pencil Sharpeners has been hitting Zess; Erinor was a useless scoundrel; Lily, as sad as it was, had been a loose end, as had everyone they'd had to kill since. Eventually the two girls agree and join in, finding flaws with all of their victims ("Full Bloom"). The musical ends as the three women cackle in the opulent dining hall, the stage fading to black as their laughter mixes with that of their children playing offstage.