Martin Dysart, a British child psychiatrist in his mid-forties, is a figure beset by existential dilemmas. Burdened with uncertainty about the efficacy of his profession and the value of his life, Dysart faces a moral quandary while treating Alan Strang. Persuaded by his friend and magistrate, Hesther Salomon, Dysart reluctantly takes on Alan's case, intrigued by the boy's unique fervor towards an imagined deity. Despite his ability to strip Alan of his destructive beliefs, Dysart is troubled by the idea of rendering the boy 'normal' at the cost of his vivid passion and creativity.

Dysart's introspection deepens as Alan probes into his personal life, particularly his sterile, childless marriage with Margaret, a Scottish dentist. This leads him to question the brisk, unexamined nature of his existence and the purpose of his psychiatric practices. Dysart's recurring nightmares of sacrificing children as a high priest symbolize his fear of robbing Alan of his intense zeal. Nevertheless, amidst his existential crisis, Dysart resolves to help Alan transcend his mental suffering, although he remains haunted by doubts.

Alan Strang, a troubled seventeen-year-old working part-time at an appliance store and Harry Dalton's stable, stands accused of blinding six horses. His upbringing is marked by conflict: his mother, Dora's, intense religious fervor clashes with his father, Frank's, staunch atheism. At twelve, Alan crafts a personal pagan faith centered on Equus, a horse god, which channels much of his mother's Christian devotion. His fervor manifests through ecstatic rides on Dalton's horses, which become sacred rituals.

Alan's internal religious turmoil peaks during his first sexual encounter with Jill Mason in Dalton's stable, the site of his religious worship. Overcome by guilt and the perceived wrath of Equus, Alan blinds the horses to protect himself from divine retribution. Initially resistant to Dysart's interventions, Alan gradually opens up, allowing Dysart to address his mental anguish with the hope of eventual healing.

Frank Strang, Alan’s father, is a mid-fifties atheist and socialist working as a printer. He dismisses his wife Dora's religious zeal and attributes Alan's psychological crisis to her influence. A staunch critic of religion, he frequently quotes Karl Marx's dictum, "Religion is the opium of the people," viewing it as 'just bad sex' responsible for Alan's troubles. His skepticism is demonstrated when he replaces a picture of Christ in Alan's room with one of a horse, inadvertently fueling Alan's equine worship.

Despite a strained relationship with Alan, Frank seeks Dysart's help, revealing parts of the night of the crime but omitting his own presence at a pornographic cinema with Alan and Jill. Frank's efforts to rationalize Alan's behavior contrast with his own failure to fully understand his son's psyche.

Dora Strang, Alan's devoutly religious mother, is in her mid-fifties. A former schoolteacher, she indoctrinated Alan with Christian beliefs, much to the disapproval of her atheist husband, Frank. Her faith, rooted in fear, presents God as punitive and omnipresent, emphasizing the sinfulness of sex devoid of spiritual love. Dora's religious teachings shape Alan's psyche profoundly, yet she is devastated by his violent actions, lamenting to Dysart that she only knows "he was my little Alan, and then the Devil came." Her sense of social superiority is reflected in her disdain for Alan’s job at a shop, which she considers 'common.'

Jill Mason, an employee at Dalton’s stable. In her mid-twenties and living with her mother, Jill learns of Alan’s love of horses and is instrumental in getting him a part-time job at the stable. She is physically attracted to Alan, communicates this to the boy one evening, and, after persuading him to go with her to see...

Hesther Salomon, a magistrate in the British legal system. In her mid-forties, she is a friend of Dysart who, troubled by the savagery of Alan’s crime against the horses and believing that Dysart is equipped to cure the boy of his mental illness, persuades the court to allow her to place Alan in the psychiatric hospital where Dysart works. It is she who persuades the already overworked doctor to take Alan as a patient. She becomes Dysart’s confidante, listening and responding to his personal and professional doubts and complaints.

Harry Dalton owns the stable where Alan commits his violent act. Once supportive and welcoming towards Alan, Dalton becomes embittered by the boy’s betrayal. He believes that Alan's actions warrant imprisonment rather than psychiatric care, viewing it as an undeserved burden on taxpayers. Dalton's initial kindness highlights the complexity of his feelings of betrayal, which mirror society's conflicted perception of Alan.

The Horseman, derisively referred to by Frank as "upper class riffraff," offers young Alan his first exhilarating horse ride. This pivotal moment is tainted by Frank’s angry intervention, dragging Alan off the horse and causing a fall. The Horseman's dismissive departure, coupled with his parting insult, leaves an indelible mark on Alan. The actor playing the Horseman also portrays Nugget, one of Dalton’s horses, symbolizing the fused nature of Alan’s experiences and beliefs.