The play is composed of seven different vignettes separated by blackouts. As the play opens, several taxi drivers sit in a semicircle. To one side stands a gunman. A large man and union leader, Harry Fatt, tells the men that a strike is not a good idea. When a man in the crowd mocks this view, Fatt calls him a "red" (slang for communist), says he is alert against their presence in the union, and claims that the reds, given the chance, will betray their fellow workers. Questions come from the crowd as to the whereabouts of Lefty, their elected chairman. Fatt reminds them they already have their elected committee present. He lets Joe, one of the workers, speak. Joe maintains he is not a "red boy", citing his status as a wounded war veteran, and discusses how if a worker expresses dissatisfaction, the union leaders label him a "red". He says his wife convinced him last week to strike for higher wages, an important theme throughout the play.
The taxi drivers remain dimly visible on stage as Edna joins Joe in their home (the scene is supposed to take place a week before the play's first scene). She tells him that their furniture, not yet paid for, was repossessed. They argue: Joe claims that strikes do not work, and that they lose money while they are on strike, while she says that while his salary barely covers rent now, soon the owners will push down their wages even more. She says his boss is "making suckers" out of the workers, and out of their families. Joe pleads that she will wake the children, but she says she only wants to wake him up. She calls his union "rotten", since they do not tell the workers what their plans are. Joe admits that they are "racketeers." When Edna challenges Joe to stand up to them, and he backs down, she tells him she's going back to her old boyfriend, since he earns a living. The taxi drivers, still in the dark, whisper words like "she will." Edna turns the subject to Joe's boss who, she says, is creating all these problems. She encourages Joe to start a workers' union without the racketeers. Joe gets swept up in her passion and tells her he is going to find Lefty Costello. Edna cheers him on. Back in the taxi driver's meeting, one of the men says that his fellow workers know better than he does, and that "We gotta walk out!"
Fayette, an industrialist, talks in his office to Miller, a lab assistant. Fayette tells Miller he is receiving a raise for his loyalty, and that he'll be switched to a new laboratory tomorrow, where he will work under an important chemist, Dr. Brenner. Miller is pleased. Fayette tells him he must remain within the building while he works on the project, which is to create poisonous gas for chemical warfare. Fayette tells him that the world is ready for war, and the U.S. needs to be ready. Miller is somewhat distraught, as he lost several relatives in the last war, including his brother. As Fayette gives him further instructions, Miller reminisces about his brother. Fayette tells him he will require a weekly confidential report on Dr. Brenner. Fayette promises higher raises, but Miller refuses to do any "spying". Fayette tells him his country needs him to do this, but Miller's mind is made up — he is willing to lose his job, as he would "rather dig ditches first!" Outraged, Miller punches Fayette in the mouth.
Florence tells her brother Irv that she needs "something out of life", and that Sid, who is going to take her to a dance, provides that. Irv warns her that both he and their mother disapprove of Sid since he makes little money as a taxi driver. Florence insists she loves Sid, and that she works hard to take care of their sick mother. Finally she buckles and says she'll talk to Sid tonight. Sid comes in, and Irv leaves. Florence and Sid pretend to be royalty before kissing. Sid says he knows he is like "rat poison" to her family. He tells her his brother joined the navy that morning, but assures her he won't run away from her. He says he can tell what she is thinking — that she does not want to marry him anymore. He laments their lowly status as "dogs" in life under the thumb of the powerful "big shot money men". He is upset that his brother, a college boy, has swallowed the "money men"'s propaganda and joined the navy to fight foreigners who are, ultimately, just like himself. Florence says she will follow Sid anywhere, but he tells her to be realistic. He turns on a record player and they dance. They stop when the music ends. He tries to make her laugh, but she buries her face in her hands, and he buries his face in her lap.
Fatt tells the taxi drivers that they have not investigated the strike issue as he has; he brings up Tom Clayton, who was in an unsuccessful strike in Philadelphia. Clayton says that his experience has taught him that Fatt is right this time. A man in the audience tells him to sit down, and Fatt tells his henchmen to "take care of him". The man runs up on stage and says that Clayton's real name is Clancy, and that he is a "rat", a "company spy", He claims that Clayton has been breaking up unions in various fields for years. Clayton keeps denying it, but the man says he knows it is true because Clayton is his brother. He tells Clayton to leave, and he does. The man is skeptical of Fatt's ignorance of Clayton's true identity.
The elderly Dr. Barnes angrily talks into a phone, upset that he has to deliver some bad news to Dr. Benjamin on an issue he opposed. Dr. Benjamin joins him. Benjamin is upset that he has been replaced for surgery on a poor woman in critical condition in the charity ward by an incompetent doctor named Leeds, the nephew of a senator. Barnes tells him that the hospital is shuttering the charity ward because it is rapidly losing money. Furthermore, they are firing some staff members, including Benjamin. Though Benjamin has seniority, he is losing his job because he is Jewish. Barnes takes a phone call and learns that the woman operated on has died. Benjamin throws down his operation gloves, and Barnes praises his idealism. Benjamin says he was not fully convinced of the ideas of radicals until now. He decides he has to work on America, and possibly get a job such as driving a taxi to allow him to keep studying. He vows to fight, though it may mean death.
A man named Agate talks to the taxi drivers, first insulting their lack of strength, then insulting Fatt. Fatt and the gunman try to detain him, but he gets away with the help of the committee men. Agate proclaims that if "we're reds because we wanna strike, then we take over their salute too!" He makes a Communist salute. While the committee men join in or take over part of his speech, Agate incites the taxi drivers with fiery rhetoric about the rich killing them off. He tells them to "unite and fight!" He says the "reds" have helped him in the past. He tells them not to wait for Lefty, who may never arrive. A man runs into the house and says they just found Lefty, shot dead. Agate yells to his fellow union men, "Hello America! Hello. We're storm birds of the working-class. Workers of the world.... Our Bones and Blood!" and urges them to die to "make a new world." He leads them in a chorus of "Strike!"