Dysart begins the act with narration, talking about how Alan showed him the way he would stand with the horse when he finished riding. After Alan goes to bed, Dysart feels as if he is alone with Equus, and Equus speaks to him, asking, "Do you really imagine you can account for Me?" This causes him to ask questions about the purpose of his career again, and he wonders what he is doing as a psychiatrist if he can never truly figure out why certain experiences, but not others, cause mental deviations like Alan's.
The nurse interrupts Dysart's musings to tell him that Alan's mother has come to visit him, but he threw his food tray at her and she is saying terrible things. They go to them, and catch the tail end of their argument, in which Dora is telling Alan not to look at her in a certain way. Dysart demands that Dora leave at once, and she does.
Dysart asks Dora never to come to his office again. He says Alan is under extreme distress, and she does not help. She says she knows that parents are not welcome here, since psychiatrists like him always blame a child's problems on them. She launches into a monologue about how Alan is just another patient to Dysart, but he is their son. She insists that despite Dysart's making them out to be criminals, they have done nothing wrong.
She defends her husband and the way they raised Alan, insisting that their household was not loveless. She says that what happened was a result of Alan himself, and no one else, and she and Frank do not deserve to be attacked for what he is. She ends by reminding Dysart that the Devil is within a person, and expresses her deep regret that the Devil came and took over her little son.
Dysart goes to talk to Alan. He tells him that he did not tell his mother anything that they talked about while Alan was under hypnosis. Alan says that nothing he said was true, anyway, and that Dysart should be locked up because of those tricks he pulls. He says he knows why he is really here—to be given a truth drug that will make him tell all.
After this, Dysart has another conversation with Hesther, exclaiming about how Alan actually believes truth drugs exist. He says he is tempted to use a placebo—a harmless pill with no effect—full of alleged truth drug to play a real trick on him. Hesther insists that he would deny everything he said afterward, but Dysart insists that underneath everything Alan does trust him, and actually wants him to know the truth.
But Martin is still not sure about what he is doing; he tells Hesther that there is nothing worse to do to someone than take away their worship, especially when Alan's worship of horses is the core of his life. He has nothing else. Hesther contradicts by reminding Dysart that because of this, Alan has been in pain for most of his life, and Dysart has the opportunity to take this pain away. Dysart says that this pain is important because it is his own pain, stemming from his intense passion that Dysart truly envies.
He admits that he is jealous of Alan Strang. He has never known worship the way Alan does. He complains about his wife, but he knows that he himself is not anything to be proud of, either. He confides in Hesther that the reason he and his wife have had no children is because his sperm count is too low. He compares the intensity of Alan's existence to his own monotonous, disappointing life, enhanced only by three weeks a year in the Peloponnese. He cannot fathom why a person like him is treating Alan for insanity. Hesther still insists that he is in pain, and wonders if Alan's stare is actually trying to claim Dysart for a new God, or at the very least a new Dad.
Dysart finds a letter from Alan in his office; Alan goes back on his statement, admitting that everything he said under hypnosis was true. Dysart asks the nurse to bring Alan to him right away.
In Act I, Dora and Frank had been driven apart by their son's crime and treatment, each trying to implicate the other in raising him the wrong way. Now, though, after much strife, Dora has begun to defend Frank and believes that neither of them should take the blame. This shift in perspective is accompanied by a fierce defense of her parenting and an insistence that she really does love her son; however, according to the shouting match she got into with Alan in scene 22, she has now begun to blame Alan, rather than herself or Frank, for what has happened.
But in reality, this is the classic nature vs. nurture debate: should blame be placed on the child's environment, or who the child innately is? Through this play, Shaffer makes it clear that the answer to this is a combination of both. Certain actions of his parents have undoubtedly enhanced Alan's pain, but much of his behavior stems from his deep, ingrained passions and his intense desire for worship, which is not something his parents could ever truly reign in.
The phenomenon called the placebo effect was first studied only two decades before the publication of this play, so naturally it was a very powerful and present idea to explore in a play that centers on the morality of psychiatry. The placebo effect dictates that a patient's symptoms can actually improve if given a useless medicine—something like a sugar pill, for instance—as long as they are led to believe that it is real, since just the concept of medicine has a very powerful mental effect. Dysart believes that a placebo truth pill would actually work on Alan, primarily because he wants Dysart to know the truth and truly desires having someone who understands the motivation behind his crime. Dysart's discussion with Hesther about placebos foreshadows the coming scenes of the play.
Another important theme that arises in this section is the idea of pain not being entirely bad, just like 'normal' is not entirely good. Dysart argues that Alan's pain comes from within him, as a result of his extreme passion, and insists that Alan could not have the passion without the pain as well. He questions his attempts to remove this kind of pain from Alan and the other children he treats, since removing the pain would inevitably remove the passion and remove their ability to feel so intensely about something. Dysart reveals that he envies Alan for exactly this reason, and would welcome some of this pain in his own life if it meant that he could worship as intensely as Alan does.
By this point, Dysart has established a pattern of trusting Hesther with his deepest insecurities. Immediately after any scene in which Dysart begins to seriously question the work he does, another scene follows in which Dysart tells all and reveals these uncertainties to Hesther. Even though Hesther rarely agrees with him on these things, she is still a powerful voice of reason to keep him on track and soothe his qualms, almost like a therapist for the therapist. But these interactions also suggest that there is something more to their relationship; Dysart trusts her with the kinds of secrets and insecurities that he has never been able to trust his wife with. In this way, Hesther has become a stand-in for Margaret.
Scene 25 ends with Hesther's postulation that perhaps Alan is not accusing Dysart of anything; rather, he is trying to claim him as a new God or a new father. Dysart quickly dismisses the former notion, but there may be some truth to the latter. As the puzzle has pieced itself together over the course of the first act, it has become abundantly clear that Frank Strang has never acted like a true father to Alan; he never even defends his son, repeatedly telling Dysart that Alan has always been strange. In his desire to fill this void and seek someone who truly understands him, Alan has turned to Dysart as a stand-in father figure.