A strike is imminent at the Sleep-Tite Pajama Factory, where the workers churn out pajamas at a backbreaking pace ("Racing with the Clock"). In the middle of this, a new superintendent, Sid Sorokin, has come from out of town to work in the factory ("A New Town Is a Blue Town"). The union, led by Prez, is seeking a wage raise of seven and a half cents an hour. Sid and Babe are in opposite camps, yet romantic interest is sparked at their first encounter. Despite cajoling from her fellow garment workers, Babe appears to reject Sid ("I'm Not At All in Love"). Meanwhile, Hines, the popular efficiency expert, is in love with Gladys, the company president's secretary, but is pushing her away with his jealous behavior. After witnessing a fight between the couple, Sid's secretary, Mabel, tries to help Hines break from his jealous ways ("I'll Never Be Jealous Again"). Meanwhile, Sid, rejected again by Babe, is forced to confide his feelings to a dictaphone ("Hey There").
During the annual company picnic, kicked off with the official Sleep-Tite Company Anthem, Prez chases after Gladys, who rejects his advances ("Her Is"), a drunken Hines demonstrates his knife throwing act (these knives are thrown at Babe), and Babe warms up to Sid ("Once a Year Day"). As the picnic-goers head home, Prez turns his attentions to Mae, who responds in the positive far more quickly and aggressively than he'd expected ("Her Is (Reprise)"). At Babe's home, Sid's romantic overtures are deflected by Babe, who makes casual conversation on tangential subjects ("Small Talk"). Eventually the walls come down between the two, who admit their love for one another ("There Once Was a Man"), but their estrangement is reinforced when they return to the factory. A slow-down is staged by the union, strongly supported by Babe ("Racing with the Clock (Reprise)"). Sid, as factory superintendent, demands an "honest day's work" and threatens to fire slackers. Babe, however, is still determined to fight for their cause, and kicks her foot into the machinery, causes a general breakdown and Sid reluctantly fires her. As she leaves, he begins to wonder again whether a romance with her is a mistake ("Hey There (Reprise)").
At the Union meeting, Gladys (Mae in the 2006 revival) performs for the rest of the union, with "the boys from the cutting room floor" ("Steam Heat"). After the main meeting, the Grievance Committee meets at Babe's house, to discuss further tactics, such as mismatching sizes of pajamas and sewing the fly-buttons onto the bottoms such that they are likely to come off and leave their wearer pants-less. At the meeting, as Prez and Mae's relationship is waning, Sid arrives and tries to smooth things over with Babe. Despite her feelings for Sid, she pushes him away ("Hey There (Reprise)").
Back at the factory, the girls reassure Hines, who is personally offended by the slow down ("Think of the Time I Save"). Sid, now convinced that Babe's championship of the union is justified, takes Gladys out for the evening to a night club, "Hernando's Hideaway," where he wheedles the key to the company's books from her. Hines and Babe each discover the pair and assume they are becoming romantically involved. Babe storms out, and Hines believes his jealous imaginings have come true ("I'll Never Be Jealous Again Ballet").
Using Gladys' key, Sid accesses the firm's books and discovers that the boss, Hasler, has already tacked on the extra seven and one-half cents to the production cost, but has kept all the extra profits for himself.
In Gladys' office, Hines, still jealous out of his mind, flings knives past Gladys (deliberately missing, he claims), narrowly missing an increasingly paranoid Mr. Hasler. After detaining Hines, Sid then brings about Hasler's consent to a pay raise and rushes to bring the news to the Union Rally, already in progress ("7½ Cents"). This news brings peace to the factory and to his love life ("There Once Was a Man (Reprise)"). Everyone goes out to celebrate—at Hernando's Hideaway ("Pajama Game").