The Royal Shakespeare Company makes its own contribution to the current monarchy debate with an exciting revival of the three parts of Henry VI at The Swan in Stratford-upon-Avon. Even if Shakespeare never ultimately questions the necessity of kingship, he shows the chaos that follows dynastic uncertainty. And, since the trilogy tours to America, you assume local audiences will see its relevance to their own constitutional crisis: that a lame-duck head of state opens the way to civil dissent.

Harold Bloom dismisses the trilogy as an exercise in Marlovian rhetoric that no longer lives. Absolutely untrue. The three plays work theatrically and appeal to audiences increasingly fatigued by the narrowing down of the Shakespeare canon. If you are going to do the plays, you should also do them as a complete set finally incorporating Richard III.

In 1963 and 1988, the RSC chopped the tetralogy into three plays dubbing them, respectively, The Wars of the Roses and The Plantaganets. Here Michael Boyd rightly follows the far better example of Terry Hands in 1978 - by presenting the Henry VI plays virtually uncut: the result is an awesome day-long event in which the actors' commitment is matched by that of the audience. Boyd and his designer, Tom Piper, have also taken radical steps to unify the stylistically variable plays. For a start they have re-aligned The Swan so that the stage-floor is an irregular oblong. Above all, they have realised the virtue of the building lies in its height. Actors are constantly scaling ladders and ropes to remind us of the vertical nature of siege-warfare. Characters descend from the flies encased in picture frames. Red and white feathers fall from on high symbolising both fluctuating fortunes, fickleness and human transience.

What is striking is how these plays change their meaning with the years. In the Hall-Barton Wars of the Roses they expressed a 1960s scepticism about the political process itself - the dominant image was of a diamond-shaped council table around which the embattled lords argued. Obviously, the plays remain acute studies in the nature of power. But Boyd treats the trilogy as, above all, an extended essay on time.

The young, callow Henry VI is haunted by his grandfather's usurpation of the throne. But the characters also exist in a nightmare present. And death is seen as an entrance as well as an exit: corpses rise up to be guided offstage by a mysterious gatekeeper only to re-appear, like the Duke of York, in the thick of battle. It's a very Eliotesque idea, that time present and time past are both present in time future.

But, however much Boyd unifies the plays visually, there's no getting away from the simple fact that Part Two is the trilogy's undisputed high point. The first Part, dealing with French wars and domestic factions, is vivid enough. But in Part Two Shakespeare displays that ability to embrace court and commoners that he was later to deploy, with even greater genius, in the Henry IV plays. At court the nobles conspire against the Duke of Gloucester; their disputes are mirrored by battles between Horner, an armourer, and his servant (delightfully named Peter Thump). We also get the Kentish rebellion of Jack Cade: as Jake Nightingale's Cade here swings on a trapeze above his followers, we are reminded that - far from being a republican - he sees himself as a legitimate claimant to the throne pre-emptively dubbing himself Sir John Mortimer.

Everyone in these plays is reaching for what they call "the glorious gold". The only person who understands the hollowness of power is the throne's occupant, Henry VI. Much has been made of the casting of David Oyelowo in the role, the first black actor to play a Shakespearean king at Stratford. Far more important is the moral strength Oyelowo brings to the part. His king may be militarily weak and politically fallible but he has the inner certainty of the believer; and when he rounds on his wife for defending her lover - "Ungentle Queen to call him gentle Suffolk" - it is with fearsome wrath. In the midst of the carnage, Oyelowo exudes a specific gravity.

I heard mumbles in some quarters that the casting could not match the great days of The Wars of the Roses. But this is, on average, a much younger company and one whose strength resides in its collective purpose. Fiona Bell, significantly doubling as Joan la Pucelle and Margaret of Anjou, moves from trull to termagant with crop-haired, iron-souled determination. Clive Wood's Duke of York watches over the action like a still, animalistic predator. Aidan McArdle as his son, Richard, whose disabilities are constantly mocked, malevolently implies the smiler with the knife under the cloak. And Richard Cordery, plumply complacent as the Lancastrian Gloucester who is Henry's protector, suggests an apple tree waiting to be axed.

Inevitably there are niggles: the emblematic scene of the father who has killed his son and the son who has killed his father is undercut by having two actors instead of four. And there are moments, particularly in Part Three, when the head spins with fatigue as the realm descends into chaos.

But, for the most part, Boyd and his team have staged the plays superbly. What they bring out is not merely the epidemic nature of throne-lust but the notion that history is a continuous inter-twining of past, present and future. It is as if Shakespeare's blood-boltered trilogy has been memorably permeated by the spirit of Eliot's Four Quartets.

• At The Swan, Stratford-upon-Avon, until February 10. Box office: 01789-403403.