The play is very surrealistic in its production and premises. As it opens, we see the boy Alan Strang very affectionately close to the horse, Nugget. The psychiatrist, Dysart, confides about his doubts and then begins to tell the story.
A judge comes and begs him to take the case of a boy who had blinded six horses one night with an iron spike. He interviews the boy, who refuses to talk (mostly), but sings television commercial jingles. The psychiatrist has a dream that night of himself as high priest sacrificing children in ancient Greece--cutting them open to read their entrails, with two acolytes waiting for a sign of weakness to sacrifice HIM.
The boy has terrible nightmares. The doctor goes to visit his parents. Television is forbidden in their household. The father is an atheist; the mother is very religious. The boy loves horses; he was given a ride by someone when he was a child, but he fell, because of his parents' hysterical and negative reactions. He tells the psychiatrist, and Dysart gives him a tape recorder so he can talk to that, perhaps more comfortably.
His parents come separately and tell him that there was a Christ picture on the wall which his dad tore down; a horse picture was substituted for it. The boy used to pretend to be a horse. He worked in the electrical shop next door and met Jill, who got him a job with her, mucking out stables and grooming horses on the weekend. He rides the horses secretly, and worships Equus, the god-spirit in all horses.
Jill wants to seduce him. After going to a porno movie with her, where he meets his father, she takes him to the Stable to have sex. He can't, because all he can see is Equus. She leaves in fear, and he blinds the horse so they can't see him.
After he relives this with the shrink, Dysart again questions what he does--how dreadful and passionless Normality is, that which he returns a man (or child) to, without their gods or strangeness--nor does he understand WHY. He himself is living in a loveless, passionless relationship (marriage). At one point he says that he would give anything to have had for one time some of the feeling the boy experienced as he worshipped the Horse.