Cock, Royal Court Theatre Upstairs

Indecision takes the characters to the point of psychic collapse and beyond in Cock, the provocatively titled Mike Bartlett play that forsakes nudity for a far more troubling collective baring of the soul. Ben Whishaw is the name draw for a run that is already pretty well sold out, but James Macdonald's production is scathingly acted across the board; this is a play best seen with someone you fully trust.

Whishaw plays John, the only properly named character in a play populated as well by M, W, and F - shorthand for their self-evident presences as Man, Woman, and Father, each of whom comes into collision with the cruelly vacillating John. At the start, John is on the outs with his older, clearly financially successful boyfriend, M (Andrew Scott), a schism that is only widened by John's burgeoning attraction to the first woman, Katherine Parkinson's long-divorced W, to whom he has ever felt a sexual pull.

Who will hold sway over John's divided, conflicted heart? Bartlett sustains that question as a tease, placing John in various face-offs with one erotic claimant and then the other until matters reach crisis point over beef, red wine, and cheesecake at a dinner party cooked by M that finds a fourth participant in F, M's liberal-minded widower father. (Paul Jesson brings a warmth coloured by understandable partisanship to the role.)

You might think that John's choice is too insubstantial to fuel a 105-minute play (no interval) in the Court's ever-elastic upstairs studio space that has here been reconfigured by the designer Miriam Buether (Annie Get Your Gun) to resemble a miniature arena of sorts in which the play's central triangle can sustain its wounding parry and thrust. The audience takes its (steeply raked) place around and above the action, a staging decision that suggests the players as so many atoms bouncing often brutally off one another. (M even refers early on to those more "together" people whose "atoms hold".)

Moving on from naturalism, Macdonald's direction frequently sets the text in direct opposition to what we see. Scenes of intimacy and even intercourse pass by with the characters fully clothed, the occasional kiss the physical extent of action that more often than not finds the variably tormented selves collapsing inward for respite. During the climactic dinner, whichever of the two younger men is not actually present takes to one of several stairways to the side of the central well where their shifts in body language say it all. At one point, John so fully folds in on himself that you feel as if his weary psyche has been punctured in accordance with a body that by that point has become the enemy.

So complete an anatomy of human breakage here extends the scalpel wielded by Bartlett in his previous (and superb) Court entry My Child to issues of sexuality only to offer up John as someone who defies labels and at his essence has no idea who - or what - he is. The implication is that he may well end up alone, a self-described "trophy" who on some level won't submit to the siren song of another, however impossible he finds it to be himself.

Bartlett's language evokes David Mamet in the way people can speak both at and around one another, not least in a remark early on from W who insists to John, "Please. We're only talking". Returning to the playhouse where she scored a definitive Masha in The Seagull, Parkinson is tremendous as the de facto sexual Virgil to the hitherto gay Dante that is John, her eleventh-hour fury at M's father taking the play's sexual debate somewhere else again. The actress makes very moving the gathering absurdity of reports of W's supposed manliness, a fiction intensified by the casting of two relatively androgynous leading men.

Playing lovers at terminal odds - or maybe not - Scott and Whishaw circle one another with a mixture of tenderness and apprehension that will be recognisable to anyone, regardless of sexuality. Hands swooping upward in accordance with his voice, Scott by the finish seems to have grown paler with worry and a fear of rejection that no amount of paternal support can put right. Whishaw, who first sprang to attention playing the father of dramatic indecision when he starred at the Old Vic as Hamlet, is less indrawn here than is his wont and rightly so. Sporting a thick head of hair toward which his hands retreat for comfort, he gives us someone physically described as "all wire" - which Whishaw is - at the point of going snap; the tightrope will not hold.

And just when you expect some sort of conclusive action or response, John takes a leaf from Hamlet. Come the play's end, M demands confirmation - a word, no more - from John and simply cannot get it. The rest, as they say, indeed is silence.