Quest - The Legend of the Green Knight, Clapham Common, London

Christmas is the right time for a dramatisation of Sir Gawain and the Green Knight. This beautiful 14th-century poem begins with the New Year festivities at King Arthur's court and the weird challenge set by the gigantic intruder: he'll submit to a blow from his own axe, provided his attacker agrees to seek him out, a year and a day later, in order to undergo the same trial. Unfortunately, in Quest - the show now playing in the big top on Clapham Common - the legend about a man who appears to have taken leave of his head is presented in a production that sometimes appears to have taken leave of its senses.

At one point, the terminally unfunny jester, Dag (Seamus Allen), says of Gawain: "If you take away his chivalry, his humility and his good looks, what have you got?" To which he gets the answer: "You." By the same token, you could ask: "If you take away Sir Gawain's poetry, its precise symbolism, its haunting sexual testing of the hero's character, its brilliant cross-cuts between three attempts at seduction and three days of hunting, what have you got?" To which the answer is: Quest.

The acting is wooden and Carolyn Spedden's script is dire. "We've been through the forest at Sherwood," Dag says. "May I just say that those men weren't very merry - at all." That's what passes for wit in these parts. Yet, despite its manifold shortcomings, there's something oddly winning about the eccentric singularity of this show.

Despite its sublime irrelevance to the plot, there's a spirited jousting tournament at Camelot, replete with beautifully caparisoned real horses, which thunder thrillingly over the mud. There's fire-eating, folk-singing, stunt-riding, sub-Cirque du Soleil acrobatics (women getting into a fetching twist with bolts of silk) and a dance routine with blazing Hula Hoops. And just in case you're getting bored by the humdrum uniformity of all that, there are some tense and flamboyant fight sequences, directed by Kate Waters. She also plays Ardala, a feisty, blond-wigged goddess, who finds herself falling in love with Damian Davis's stoical Gawain, as she assists him in his quest.

Too much time is devoted to the hero's adventures on the way, where he encounters everything from spindly green Spriggans on tottering stilts to tutu-wearing pixies with a mental age of seven, who sprinkle snow over the audience. There's no sense whatsoever that the Green Knight is a double-edged figure (because there's no equivalent of the lord in the poem who offers the hero hospitality). He's just an out-and-out villain (though his galloping away with his head under his arm is a breathtaking coup de théâtre).

Still, I'm glad I went. If you're thinking of making the trip to Clapham, rest assured that you won't see anything quite like this again any time soon.

To Monday (020-7482 0115)