We see for the first time the direct conditions of the firemen in the stokehole: stripped to the waist, they labor strenuously to feed the furnace with coal in an atmosphere full of menacing shadow and light. Instead of the human noise of the forecastle roars the inhuman sound of the machine. While Paddy bemoans his own weakness with age, Yank works at the top of his form, as though effortlessly, and shouts brusque encouragement to his fellow workers. When he hears the whistle of the unseen engineer, Yank responds with a proud contempt, shoveling coal in at an even more incredible rate; he means to show his employers that they are insulting his strength by signaling, with the whistle, that they want the work to be done faster.
As Yank sets the example, the other men chime in with him and stoke each other's motivation. However, yet another blow of the whistle sets off Yank's temper. He works at a devilish pace and begins to yell threateningly at the engineer.
It is at this emotional height that the second engineer enters the stokehole bringing an already intimidated Mildred. The other stokers see the girl and stop their work, transfixed by the sight of her pure white dress and feminine form. Yank, however, is unaware of her presence and only continues to work himself into a greater frenzy, bordering on the animalistic.
At a climactic moment, he turns around with his most threatening look, expecting to frighten his employer with his raw strength, but is shocked wordless to find himself face-to-face with Mildred, who is utterly horrified at the sight of him.
As Mildred faints into the engineer's arms, she cries for the latter to take her away. Yank, struck by a feeling of deep and unidentifiable confusion and embarrassment, lets out his rage by throwing his shovel at the door the two leave through.
Back in the forecastle, the firemen are taking a break from their work. Whereas the rest shower to clean up from the grime of the stokehole, Yank remains sitting on a bench hunched over and plunged into deep introspection, making a position just like Rodin's famous sculpture “The Thinker.” The other workers voice their concern about him, but Yank waves them away, saying "Lemme alone. Can't youse see I'm tryin' to tink?"
Paddy gives an explanation: he says that Yank is in love. Yank furiously denies this, saying that it is hate that he feels. At this point, Long jumps up and gives another socialist harangue about the situation: he decries the engineers and the management of the ship for allowing Mildred to come see them in the stokehole as though they were animals in a zoo. Although Yank finds him ridiculous, Long's comments, along with Paddy's and the other workers' making fun of him for his strong reaction, drives him into a second frenzy. He makes to charge out of the forecastle to find Mildred on the ship and exact his revenge, but the others restrain him.
Yank's sudden, momentary encounter with Mildred in the stokehole constitutes the primary traumatic moment in the play and in Yank's life that drives the rest of the plot. Whereas Yank has been the same confident and contemptuously powerful man up to that point, always demonstrating his worth in his outward actions and earning correspondingly the admiration of his fellow workers, the shock of a certain feeling of implacable shame drives him into himself and earns him the kind of derision previously unthinkable.
O'Neill underlines the momentousness of this change by an ironic reversal of Yank's own words; whereas in the first scene he used the phrase "Can't you see I'm trying to think" as a sarcastic denigration of contemplation (and by contrast a boast about his physical prowess), when he says it again in this scene, he does so with a markedly different tone that the other men do not fail to notice: "They are silent, puzzled by his sudden resentment at what used to be one of his jokes. YANK sits down again in the same attitude of ‘The Thinker’” (60).
Yank only reacts so violently to Mildred because of a certain double psychological switch that literally happened behind his back in Scene Three: the whistle he thought was a signal to work harder was in fact a command to stop; and the engineer whom he cursed so violently and prolifically turned out to be a young woman. As regards the latter, there is actually something moving truthful about Yank's confusion: Mildred, as we learned in Scene Two, is not only daughter of a big capitalist but herself a bourgeois poser (albeit a well-intentioned one) who seems to be primarily driven by a kind fo detached and ironic curiosity to see the stokehole.
That is to say, she represents the unscrupulous employment that Yank rails against just as much as does the engineer with his whistle. To go a step further, we might even identify the shocking whiteness of her dress—whose visual vividness conveys the force of the emotional shock—with the piercing sound of the whistle. Crucial to this identification is the banter that Mildred made with the second engineer to embarrass him: she had flaunted her possession of many similar dresses in response to his concern about her dirtying the one she wears. Both whistle and replaceable white dress represent to the working men a kind of upper-class contempt for them and an exercise of power or wit to keep them distant.
Mildred does not appear again in the play after she “whimperingly” says to the engineer—and significantly, not to Yank or the other workers, with whom she at this point has no interest speaking to, if indeed she had any to begin with—“Take me away! Oh, the filthy beast” (58). However, her quick in-and-out in Scenes Two and Three, essentially serving the function of setting up the traumatic collision between Yank and his subsequent development as a character, does not rob her of her own dramatic interest. It is one of O'Neill's talents as a dramatist that he is able to invest secondary characters, even in a play clearly focused on a single protagonist, with convincing and deep personalities of their own.
Just as Yank reacts in a way that betrays his own and others' expectations of him, so does Mildred. Although we may see some truth in her aunt's accusation of her being a “poser” given her inability to stand the situation and what we may assume to be her loss of interest in the firemen after this incident, we may be inclined to take her for her word that she actually has an emotional commitment to breaking class boundaries. Whether this drive goes beyond the curiosity and wit that she shows so strongly in her conversational dominance over the second engineer may depend on how the play is staged; either way, her character possesses, as does Yank's, a certain ambivalence that is not really brought into the foreground until she is faced with raw reality.
As a final note, Yank's and Mildred's encounter is perhaps the only moment of genuine (albeit traumatic) human encounter in the entire play. Pre-Mildred, Yank is dismissive and contemptuous of others; post-Mildred, he becomes almost paranoid in his suspicion of others.