A flotilla of wimples sails on to the Courtyard stage and sets the tone for the next three and three-quarter hours. (This is not a show for day trippers; you would miss the last train out of town.) Shakespeare can easily be staged in modern dress with guns doing service for rapiers, but Arthurian legend demands wimples, armour, chainmail and broadswords. The most modern touch comes when the adolescent Mordred is shown hoicking his aumuse up around his face like a mediaeval hoodie.

The material dictates much of the look and feel of Gregory Doran's production. In his adaptation of Thomas Malory's 21-book cycle (published by William Caxton in 1485, 15 years after the author's death), Mike Poulton cleaves as closely as is practicable to the original language. Many archaisms have been excised, but some remain: the Knights of the Round Table are injoined by Arthur "that you never do outrageosity nor murder", and Sir Gareth is "slain in the hurtling as Sir Launcelot thrang in". This also, loath though I am to concede it, makes an inadvertent case for Received Pronunciation onstage. My countryman Jonjo O'Neill is an excellent actor and here he well embodies the flawed virtue of Launcelot, but Malory's language sits awkwardly with his Ulster twang, especially since in other cases regional accents are used to implicitly comic effect, as with Peter Peverly's Geordie Mordred. The style is always declamatory... surprisingly, even more so in the final phase despite a succession of battles. It feels not simply epic but sluggish, especially as Terry King's fight direction here tends often towards the stylised.

The ensemble cast of 21 show a mighty commitment. Sam Troughton as Arthur travels without major makeup from boyish squire through to exhausted and ageing king; Forbes Masson's Merlin has a touch of Nicol Williamson's gloriously bonkers performance in the role in John Boorman's film Excalibur; and there are appearances by current RSC assets such as Noma Dumezweni, Christine Entwisle and a cameo from Mariah Gale as a damosel who expires for love of Launcelot. (Aye, aye, that's Shalott.)

But Malory's work does not possess adequate narrative drive: it rambles through a Round Tableful of knights, a slew of adventures and quests, a flange of handy hermits and so forth. Even the Grail quest and the final strife, which constitute most of the latter two of the evening's three acts, feel baggy. And yet a version which cut an hour or so might seem paltry, or spreading the material across a diptych profligate. This is absolutely the kind of project the RSC should periodically take on; it's just that sometimes it doesn't pay off.

Written for the Financial Times.