The Hairy Ape opens in the firemen’s forecastle of a large ocean liner, where Yank and his coworkers spend their time when they’re not shoveling coal into the boat’s furnaces. The room is loud and rowdy, the many stokers drinking, singing, and shouting. Amidst the uproar, Yank sits and listens to his fellow stokers. He is the “most highly developed individual” in the cage-like forecastle, where the men resemble Neanderthals. The stokers get louder until, finally, Yank tells them to be quiet because he’s trying to “think”—an idea that makes all of them laugh. Nonetheless, everyone heeds him for a moment, but soon they’re back to their chattering. One stoker belts out a song about a woman waiting for him at home, a sentiment Yank scorns. He goes on to say that the only home these men have is the ship itself.
Hearing Yank’s speech about home, a drunken stoker named Long stands up and expresses a number of communist values, describing the ship as “hell,” and saying, “All men is born free and ekal.” Continuing in this manner, Long says that the “Capitalist clarss” has made the stokers into “wage slaves,” but Yank tells him to be quiet, calling his ideas nothing but “Salvation Army-Socialist bull.” Instead of agreeing with Long’s critique of the inequity that arises under capitalism, he insists that the work he and his fellow stokers do in the engine room has nothing to do with the rich people in the first cabin. Indeed, he upholds that rich people aren’t strong or brave enough to work as stokers, arguing that only true men have enough “noive” (nerve) to work in the hell that is the stokehole. This, he says, is where he “belongs.”
“We belong to this, you’re saying?” says a stoker named Paddy, an older Irishman. Paddy disagrees with Yank and waxes poetic about the past, when he used to sail above deck on beautiful ships with the wind in his face. Arguing that this kind of work was hard but rewarding, Paddy frames the stokehole as exploitative and unfulfilling. Yank responds harshly to Paddy’s ideas, calling him “crazy” and moving toward him violently, though he backs off and decides he doesn’t deserve a beating because he’s “too old” to understand what it means to work in the stokehole. “I belong and he don’t,” Yank says to his coworkers, insisting that Paddy has lost his nerve. Yank then says that he likes being at the “bottom” of the entire world, since he thinks his job is what makes everything run. “I’m de end! I’m de start! I start somep’n and de woild moves!” he says.
Two days later, Mildred—the daughter of a steel tycoon—sits above deck with her aunt and looks out at the sea. The two women talk about Mildred’s desire to see how “the other half lives.” Her aunt criticizes her for wanting to help the poor in ways that only make them feel “poorer,” but Mildred pays no heed, instead insisting that she’s only trying to “be some use in this world.” She says she’d like to be “sincere” for once, though she doesn’t know if it’s possible. She then posits that the wealth she has inherited has left her with “none of the energy, none of the strength of the steel that made it.” In search of this kind of “energy,” she has decided to take a tour of the engine room. Her aunt is incredulous, but Mildred is set on seeing the stokehole. She even told the captain—who didn’t want to let her do this—that her father is president of Nazareth Steel and owns the ocean liner, and so he had no choice but to let her do what she wants. When an engineer finally comes to escort her below, he urges her to change because she’s wearing a white dress. He tells her she’s sure to rub against grease on their way down, but she only says, “It doesn’t matter. I have lots of white dresses.”
In the stokehole, the men are busy shoveling coal into the furnaces in intervals marked by the sound of a whistle, which an engineer blows from an unseen perch above. When the whistle blows, they throw open the furnaces and are blasted with heat, which is why they need frequent breaks. Although most of the men complain that the engineer blows the whistle too quickly, Yank criticizes them for being weak, urging them along until, finally, he too becomes angry. Before long, he turns around and screams into the darkness, the shovel raised over his head as he yells violent threats at the engineer with the whistle. At this point, he notices that his fellow stokers are all looking at something behind him, so he whirls around and crouches with the shovel held above his head. What he finds is Mildred standing there in her white dress, staring at him; “As she looks at his gorilla face, as his eyes bore into hers, she utters a low, choking cry and shrinks away from him, putting both hands up before her eyes to shut out the sight of his face, to protect her own,” reads O’Neill’s stage direction. “Take me away!” she shrieks. “Oh, the filthy beast!” With this, she faints, and the engineers at her side carry her away. As he watches her go, Yank “feels himself insulted in some unknown fashion in the very heart of his pride,” and says, “God damn yuh!”
Back in the forecastle, Yank sits in the position of Rodin’s “The Thinker” while the other stokers speak loudly. Once again, he tells them to be quiet because he’s trying to think, and as they try to guess what’s going on with him, Paddy suggests he’s fallen in love. “I’ve fallen in hate, get me?” Yank says. Long gets up and insists that Mildred’s presence in the stokehole only further emphasizes the divide between the “Capitalist clarss” and the workers, and he tries to get Yank to see that they can—“as voters and citizens”—make a difference if only they finally unite. Agreeing with Long, Paddy says that it was as if Mildred had “seen a great hairy ape escaped from the Zoo.” Hearing this, Yank vows to take his revenge.
Three weeks later, Long takes Yank to Fifth Avenue in Manhattan. “We’re trespassers ’ere,” Long explains. Proletarians keep orf the grass!” Failing to grasp that he most likely won’t be able to find Mildred herself, Yank grows impatient, but Long tells him to wait until church lets out, at which point rich people stream over the sidewalks. Giving up on the idea of finding Mildred, Yank tries to pick fights with wealthy passersby, but nobody even acknowledges him, and Long decides to go home because he knows Yank is going to get them in trouble. Finally, after accosting multiple people, Yank slams into a wealthy gentleman. The man calls for help and “a whole platoon of policemen rush in on Yank from all sides.”
In jail the following night, Yank sits in the position of Rodin’s “The Thinker” and likens his cell to the cages that animals are kept in at the zoo. His fellow prisoners hear him say this and they make fun of him. He then tells them about Mildred, explaining that she’s the daughter of a wealthy man. A man in the nearest cell suggests that Yank “join the Wobblies” if he wants to find Mildred. When Yank expresses his confusion, this prisoner reads him a newspaper article about the Industrial Workers of the World union (IWW). The article is from a speech made by a senator who publicly warned the senate about the “wobblies.” Quoting the newspaper, the prisoner reads, “They plot with fire in one hand and dynamite in the other. They stop not before murder to gain their ends.” Although the senator meant these words metaphorically, Yank takes the ideas literally and decides to join the IWW when he gets out of prison, thinking this will help him get back at Mildred and her family. In a determined rage, he starts shaking the bars of his cell, and when a guard comes to tell him to stop, he rips the cage apart. Frantically, the guard yells for backup and sprays a hose into the cell as the curtain closes.
A month later, Yank visits an IWW branch and asks if he can join. The secretary who works inside gives him pamphlets and welcomes him in a friendly manner. However, Yank keeps alluding to the idea that the IWW has secret schemes to blow up companies like Nazareth Steel, and this aggravates the secretary, who orders the men hanging around the office to throw Yank out. Sitting in the street outside, Yank says, “So dem boids don’t tink I belong, neider.”
The next day, Yank goes to the Zoo and talks to a gorilla about how he doesn’t “belong” anywhere. “Yuh get me,” he says, accepting the idea that he is the “hairy ape” Mildred thinks he is. He tells the gorilla he will let him free, saying that they can stick together and wreak havoc on the people who want to put them both in cages. When he lets the gorilla out, though, the animal squeezes him so hard that his back snaps, and then the large creature tosses him into the cage. “Even him didn’t tink I belonged,” Yank says. With his dying breath, he says, “Ladies and gents, step forward and take a slant at de one and only—one and original—Hairy Ape from de wilds of—” With this, he dies as the monkeys in the surrounding cages start “chattering.”