"Jerusalem". Magdalen Garden

Four years ago, the Daily Mail declared, with blind, patriotic inaccuracy, that Magdalen College had cancelled St George. The reason they gave was that the college had decided not to celebrate the patron saint of England's special day with a dinner. The Mail neglected to mention that Magdalen has virtually never had a St George's Day dinner in the five centuries of its existence. Nor have most of the other colleges. But for a few days the President was besieged with outraged emails from retired colonels, which she replied to with her accustomed patience and clarity.

Now Magdalen has got its own back. In staging Jez Butterworth's magnificent, tawdry, poetic, potty-mouthed masterpiece Jerusalem in the President's Garden, the college has stuck two fingers up at the Mail, while simultaneously finding a deeper and more meaningful commemoration of St George than any dinner could possibly evoke, even if it served dragon steak.

Jerusalem takes place on St George's Day, when the good, and not-so-good, folk of Flintock in Wiltshire hold an annual fair, crown a May Queen and go a-Morris dancing. But we don't see any of that. Instead, the action is focused on the filthy environs of Johnny 'Rooster' Byron's rundown caravan in the woods, and Byron himself, a wastrel, alcoholic mythomaniac who claims, while selling drugs to the local teenagers, to be an immortal spirit. He tells stories of meeting giants, of being born able to speak, of his ancestors' dead bodies refusing to decompose in their graves. He has the bravado of the artful dodger and the charisma of the Pied Piper.

But the captivating thing about Jerusalem is that Rooster might actually be telling the truth. Butterworth's argument is that the spirit of England isn't noble, chivalrous and meekly Christian: it's chaotic, hedonistic and still in touch with mythologies that stretch back far further than Jesus.

Billy Skiggs and Billy Hearld (collectively known as The Billys) have produced a version of this play that reveals every dark corner of its amoral heart, and also, despite the broken bottles, crisp wrappers, empty cans and badger shit, manages to be ethereally beautiful.

The magical atmosphere that envelops this production is down to many factors, but chief among them must be the way the Billys use the Magdalen President's Garden. The vast majority of garden plays don't have anything intrinsically botanical about them. They're plonked on a lawn to celebrate summer, and they're frequently defeated by the space: inaudible, plagued by passing croquet players, or simply having nothing to do with the outdoors.

Jerusalem, on the other hand, is an inspired choice for a garden. In-theatre productions have to construct Rooster's forest on a stage. Here, it is in its natural environment. The leafy archways leading to the Lodgings form perfect, tree-like entrances, the bushes and shrubs become willing pieces of scenery, and the evening blackbirds provide a soundtrack that sings, 'This is England'. It's a set that truly finds tongues in trees, books in the running brooks, sermons in stones, and whizz in every thing. The Billys have even imported a full-size, skuzzy caravan, Rooster's undesirable residence, which hulks half-seen in the background, like a remnant of Mark Rylance's West End production.

The other ingredient that makes this show a towering achievement is a collection of performances from a cast who nail the comedy, danger and magic in equal measure. Acting characters who are off their heads on drugs or alcohol is no easy task: they often come out mumbling and tiresome. But Ezana Betru, Orla Grist, Nicholas Crossa (I don't normally list names but they damn well deserve it), Peregrine Neger, Hannah Suckling, Sanaa Pasha, and Alexander Lloyd-Elliott turn Rooster's posse of pissed pals into a group of fascinating, tragic individuals whose failed-but-funny lives fuel the show with mournful comedy. If you passed these people in a woodland glade while out walking the dog, you'd probably creep away in terror. Butterworth forces us to hear them and accept them as humans. 

But it's the role of Rooster himself that makes or breaks Jerusalem. It's a gigantic part: conniving, petty, mystical and pagan. The actor who takes it on has umpteen monologues to recite, and somehow has to give audiences a glimpse of the eternal. Adoloras is more than up to the task. He chats in a slurred, offhand Wiltshire brogue, while capturing the entire garden in a charismatic spell. Some of his lines hit you like a prophecy of doom ('I said, "Mother, what is this dark place?" and the response, "‘Tis England, my boy. England."') At one point he forces his ex-lover Dawn to look in his eyes, and then asks her, 'Do you see it?' She can reply only 'Yes'. And I think we see it too.

Adoloras's performance is all the more astonishing when you realise that every day this week, while acting Rooster in the evening, he has been taking Finals exams during the day. As I may have said before, only in Oxford.

Jerusalem is often cited as one of the greatest plays of the 21st century. In the magic of Magdalen garden you can see why. At the very moment that Peregrine Neger's Professor calls for an explanation, the bells of the Great Tower answer with their ancient chime. It may be coincidence, but it feels like we're in sync with something beyond our control. This production invites us to keep that connection alive.