In the play and film, psychiatrist Martin Dysart is called to investigate the case of a stableboy named Alan Strang. Alan, out of a religious and sexual fascination with horses, savagely blinded six horses with a metal spike. As he examines the boy, and his fascination, Dysart starts to have doubts about whether he can really help him, or whether turning people to a "normal" way of thinking is always the right thing to do.
The film features examples of:
- Bad Dreams: Dysart's Greece dream in scene 5, and Alan's frequent dreams concerning "Ek" or Equus.
- Bestiality Is Depraved: Alan can't distinguish between affection for horses and sexual attraction.
- Critical Psychoanalysis Failure: Alan's Equus hallucinations are passed to Dysart.
- Crisis of Faith: Word of God says Dysart's increasingly sour and skeptical attitude toward his own job is a secular version of this; early in the play he lightly dismisses it as "professional menopause", but it becomes clearer — especially toward the end — how deep his doubts really go.
- Dysfunctional Family: The Strangs. Frank and Dora are of wildly differing personalities and perspectives, and their strongly conflicting views of how to treat their son only contribute to his psychosis.
- Eye Scream: The blinding of the horses.
- Full-Frontal Assault: Alan's attack on the horses is done in the nude.
- History Repeats: As Doctor Dysart prepares to 'cure' Alan, he reveals that he himself has now become plagued by visions of Equus.
- Male Frontal Nudity: The script calls for the actor playing Alan to appear naked on stage. Predictably, the production starring Daniel Radcliffe spawned countless jokes about Harry Potter showing his "wand." Curiously, the script only calls for the actor playing Alan to mime stripping, never actually requiring any nudity, and indeed Peter Firth, who originated the role on stage, did so there - before playing the scene naked and full-frontal - rather more impressively as well - in the film version. Radcliffe decided to follow Firth's lead.
- Minimalism: The original play features, among other aspects, only one setting and actors or dancers in lieu of horses; very much averted in the film version, naturally.
- Never My Fault: Dora, who says the reason her son's blinding the horses is because he's possessed by the devil.
- Not So Different: Alan and Dysart appear separated in a multitude of ways, yet it eventually becomes clear that the staid, predictable Dysart is himself obsessed with the kind of raw passion that Alan experiences, as evidenced by his monologues and bizarre dreams about ancient Greece - but is shown as too afraid to grasp it until his final lines.
- Shout-Out: There's a reference to "standing in the darkness, stabbing at heads" which seems to refer to the play Agamemnon by Aeschylus.
- Significant Double Casting: Traditionally the actor who plays the rider who gives Alan his first horseback ride also plays his favourite horse Nugget, which highlights Alan's confusion over his sexuality.
- Very Loosely Based on a True Story: The play was inspired by a headline of an actual horse blinding; Peter Shaffer then devised the story of his play from the ground up.
- What Happened to the Mouse?: Or rather, the horses—we're not told their fate after Alan blinds them.