The firemen, workers who shovel coal into the engine of a Transatlantic Ocean Liner, sit in the forecastle of the ship drinking and carrying on with each other. They are an hour out of New York City and have seven more days aboard ship. The men are burly and muscular. Yank, the fiercest looking of the men, sits in the foreground quietly. Whenever Yank speaks the men immediately hush. Yank asks for a beer and the men immediately give one to him. As Yank and the men drink, Yank remains in control as the leader of the group. Yank and the men joke about thinking as they drink. Yank, in a joke repeated during the play, tells the men to be quiet because he is trying to "tink." The men mockingly repeat after him, "think" and then erupt into a chorus of "Drink, don't think!" Cutting through the general mayhem, a drunken tenor sings a tune about his lass at home. Talk of home outside the ocean liner infuriates Yank and he tells the tenor to be quiet. Long, quite drunk, stands up and makes a Marxist speech, preaching to the men that if the ship is home, their home is hell and the Upper Class put them there. Yank tells him to join the Salvation Army and get a soapbox. Paddy, a wise, older fireman tells the men that life on an Ocean Liner is hell by comparison to his life on a Clipper Ship. Paddy reminisces about the freedom he enjoyed, the purpose he had and skill for which he was valued. Yank tells Paddy that he is dead, "living in the past of dreams" and glorifies his own job as the strength of the ship's speed and force.

Mildred and her Aunt lounge on the promenade deck of the Ocean Liner. Mildred and her Aunt discuss Mildred's need to do service for the poor. Mildred worked with the poor in Manhattan's Lower East Side and is currently on her way to do more service projects in Europe. Mildred's Aunt characterizes Mildred's service as "slumming" and does not understand why she has to do it internationally. Mildred's Aunt tells Mildred that her service work just makes the poor feel poorer. Mildred is currently waiting for the second engineer to take her down into the stokehole. Mildred told a lie that her father, the president of Nazareth Steel, has given her permission. When he arrives, the second Engineer escorts Mildred, clad in a white dress she refused to change out of, down into the stokehole.

Yank and the men are hard at work shoveling coal in the noisy stokehole at the opening of Scene Three. Yank leads the men at work. The men take a break and an anonymous whistle-blower overhead in the darkness commands the men to keep working. In a rage, Yank screams up at the whistle-blower. Yank suddenly realizes that the men have stopped working. Still fuming, Yank turns to face Mildred. At the sight of Yank, Mildred whimpers for the men to take her away from the filthy beast and faints into the arms of the engineers.

The men have again gathered in the stokehold in Scene Four. They replay and rehash the Mildred scene and mock Yank, the "filthy beast." Paddy tells Yank Mildred looked at him like he was a big "hairy ape." Infuriated, Yank lunges toward the door to find Mildred, but is restrained by the other Firemen.

Yank and Long have traveled to 5th Avenue in New York City in Scene Five. Long means to show Yank that all upper class people are like Mildred. Yank tries to attract attention to himself by bumping into people and accosting a young woman, but receives no response but "I beg your pardon." Finally, Yank is arrested because he makes a Gentleman miss his bus. Yank is imprisoned on Blackwell's Island and converses with the other prisoners in Scene Six. The men tell him that if he wants to get even with Mildred and her father's company he should join the Wobblies or the Industrial Workers of the World. Yank realizes that Mildred's father built both the physical and metaphorical cage he is trapped in. In a fury, Yank actually bends the bars of his cell, but is restrained by the guards.

Yank visits the local I.W.W. in Scene Seven, but is rejected because the Secretary thinks he is a governmental spy. Yank's radicalism, willingness to blow things up and preoccupation with "belonging" make them suspicious of him. Yank is thrown out on the street. Yank spends the night at the Battery and the next morning visits the Monkey House at the Zoo. In Scene Eight, Yank attempts to befriend the ape. He tells the ape that they are alike—both caged and taunted. Yank believes he and the ape belong to the same club and calls him brother. Yank releases the gorilla from his cage and approaches the ape to shake his hand. The gorilla springs on Yank, crushes Yank with his massive arms and then tosses Yank into his cage. Yank dies in the gorilla's cage.