Hesther Salomon, Eileen Atkins a magistrate, asks her platonic friend Martin Dysart, Richard Burton a disillusioned psychiatrist who works with disturbed teenagers at a hospital in Hampshire, England, to treat a 17-year-old stable boy named Alan Strang, Peter Firth after he blinded six horses with a sickle.
With Alan only singing TV commercial jingles, Martin goes to see the boy's parents, Frank Strang, Colin Blakely and Dora Strang, Joan Plowright, the non-religious father and his Christian fundamentalist wife. Dora had taught her son the basics of sex and that God sees all, but the withdrawn Alan replaced his mother's deity with a god he called Equus, incarnated in horses.
Frank Strang discloses to Martin that he witnessed Alan late at night in his room, haltered and flagellating himself, as he chanted a series of names in Biblical genealogy-fashion which culminated in the name Equus as he climaxed.
Martin begins winning the respect and confidence of Alan, who shares his earliest memory of a horse from when he was six and a man approached him on a horse named Trojan. The man took Alan up on Trojan, which the boy found thrilling, but his parents reacted negatively and injured him taking him off the horse. Martin also meets the stable manager, who reveals Alan secured his job through another employee, Jill. Devastated at the horses' injuries she indirectly caused, Jill has taken medical leave.
Eventually, Alan admits to Martin that he would secretly take horses away from the stables at night to ride them nude, chanting prayers to Equus until he reached orgasm, after which he caressed them lovingly.
Martin envies the boy's passionate paganism, in comparison to his own empty life, where he has ceased intimacies with his wife and is plagued by nightmares of ritualistically slaughtering children in Homer's Greece, wearing the Mask of Agamemnon.
Given an aspirin serving as a placebo "truth drug", Alan further reveals that one evening Jill tempted him to go to a pornographic film at a local cinema, where he was shocked to see his father, who forced him to leave. After Alan went back with Jill to the stables, she stripped and offered him sex but he was unable to perform and, although she was sympathetic, told her to leave. Naked, and tormented that Equus sees all and is a jealous god, he blinded the horses.
Martin is left troubled by the fact that he can treat Alan to take away his pain but in the process will deprive the boy of his passion, leaving him as emotionally neutered as Martin himself.
