Pedro, the Great Pretender, Playhouse Theatre, London

Comedy dates quickly and is notoriously difficult to translate. Being a 400-year-old Spanish comedy, Pedro, the Great Pretender, by Miguel Cervantes, is handicapped from the outset. In this case, the difficulties are compounded by an uneven, episodic structure - 17th-century Spain was, after all, the birthplace of the picaresque novel. It is interesting to discover from the programme that the play was not performed in Cervantes' lifetime: compare Shakespeare, Cervantes' contemporary, whose plays had all been thoroughly tested on stage before they appeared in print.

Under the circumstances, Philip Osment's translation and Mike Alfreds' production - part of the RSC's Spanish Golden Age season - are minor miracles of performability; but that is not quite the same thing as a satisfying evening's theatre.

Pedro, in a performance of effortless authority by John Ramm, is a wily but fundamentally good-hearted rogue, known and liked for his sharp wit and common sense. The play falls roughly into three sections. It opens in a pastoral vein, with Pedro sorting out the difficulties between shepherds and their sweethearts, in between comic interludes with his buffoonish master, the local mayor. Along the way, Pedro tells the story of his life, how he has learnt his cunning through employment with card-sharps, blind beggars and cowardly knights.

The emphasis shifts to Pedro's cunning, as he joins a band of Gypsies and sets about conning a grasping widow out of her fortune. Meanwhile, the mayor introduces a troupe of transvestite boy-dancers, with which he hopes to catch the eye of the king. In the final section, the king turns up, and Pedro's exploits take second place to the story of the beautiful, haughty Gypsy girl Belica, whose delusions of grandeur turn out to have some foundation after all.

Osment's translation is adroit in its use of shifting metres and its profusion of internal rhymes and assonances, though at times it feels as though he has dipped at random into a dictionary of English idioms ("That mayor is just the limit!"; "It's clear as day").

The costumes, by Johanna Coe, are similarly unmoored in any one period - the peasants sport late-Victorian waistcoats, watch-chains and stiff collars, while royalty is dressed in a sort of school-play medieval. Ilona Sekacz's music uses vaguely Iberian rhythms to pleasant though not striking effect.

The large cast has no real weak points, and some of the supporting performances are particularly strong (notably, Claire Cox's snooty Belica). The effect is jolly, and when the production was first staged at the more intimate Swan in Stratford, I imagine that that overwhelmed any defects. But it feels marooned in the open prairies of the Playhouse. A telling moment came when two actors rushed on to mop up water that had been chucked over the stage during a dance. "Health and safety," Ramm confided to the audience - and got easily the biggest laugh of the evening.

To 12 March (0870 060 6631)