Theatre / La Grande Magia, National Theatre

It's not easy to sum up the rich mixture of emotions prompted by the first half of La Grande Magia, but if you had to choose one word the best one would probably be "perplexed". Richard Eyre's production doesn't simply confront you with puzzles about what is illusion and what is reality - heck, we've all come across those in the theatre before - but about what sort of play this is.

It seems clearcut enough for the first few minutes: essentially, a naturalistic play about the guests at a hotel on the Italian Riviera in 1948 (Anthony Ward's design seems to place it a good 20 years earlier); in particular, it's about the poisonous relationship that exists between a jealous, ageing Di Spelta (a primped, pompous Alan Howard) and his beautiful younger wife. Everything changes with the arrival of Professor Marvuglia - an excellent Bernard Cribbins, at once seedy and mysterious - a stage magician who hints at having larger, darker powers.

Actually, Marvuglia is an unsuccessful conjuror, who has been employed specifically to perform the Vanishing Lady trick on the young wife, to enable her to meet her lover. When she fails to reappear, having eloped with the lover, Marvuglia tries to cover up by persuading Di Spelta that he is himself in the grip of an illusion and that they are still on stage at the hotel.

So far, so good. What complicates things is not so much the shifting of the boundaries between the real and the imagined, but some peculiar shifts in tone: at home Marvuglia plays with an applause machine that allows him to bask in illusory adoration (is this a swipe at fascism?); the arrival of an absurdly posturing, slapstick police inspector (played by David Ross) pushes the action sharply into farce; grief intrudes when a young girl dies, and mourners process through the room where Marvuglia is still desperately playing on the credulity of the cuckolded husband.

The result is a sense that Eyre has elaborated De Filippo's conceit in ways the author didn't intend. The second half tends to confirm this: four years on, di Spelta has retreated into a dream world. Strangely, though, the effect of this is to leave you grateful for the oddities of the first half; because while this is unsettling, and Howard's broken, petulant madman is brilliant, it's not especially surprising. It may be that Eyre has given the play some complexity that it doesn't really hold. I don't, in the end, know quite what to think about play or production: often irritating, but at times also funny, disturbing and moving. I am left with a gnawing suspicion that there's less going on here than meets the eye.

Lyttelton Theatre, London SE1 (0171-928 2252)