On Thursday I went to the first night of Tchaikovsky's opera Eugene Onegin at the Royal Opera House in London. Pushkin's plot turns on a couple of crucial scenes. In one of them, the poet Lensky is infuriated by the sight of his fiancée Olga flirting with Eugene Onegin at a party, and challenges Onegin to a duel (in which Lensky is killed). Another is when Onegin, having spurned the inexperienced young Tatiana years before, attends a grand ball in the city, and is thunderstruck by the dazzling sight of Tatiana, now married to a prince and the focus of all eyes. He realises that he loves her after all.

The Guardian review wasn't convinced by the production, but the reviewer didn't mention the thing I found most disconcerting. For some mysterious reason the director had decided to underplay both the scenes mentioned above. First of all, Eugene Onegin did not flirt with Olga, but merely danced with her in a wholly decorous way. It seemed well within the bounds of acceptable behaviour. So because we had seen nothing infuriating, it was almost comical when Lensky reacted with fury. Did the director mean him to look ridiculous?

Then the final act opened, not onto the scene of a glittering ball as described by the author, but onto a winter park. Sombrely-dressed people in dark overcoats and fur hats moved slowly about as if they had come from a funeral. Incomprehensibly, they did so to the jaunty sounds of a polonaise.

You could feel the whole audience thinking, '......Eh?'

As I watched I began to wonder whether this failure to flirt, followed by no grand ball, was a directorial double-bluff or perhaps a postmodern gambit. Was it a case of 'spot the deliberate mistake', or was something subtle being attempted? Did the director hope that less would be more, that not doing something might speak louder than doing it? Was the non-flirting a sign that flirting is nothing? Was there meant to be a cognitive dissonance between the polonaise and the non-dancing, and if so, what did it signify? It was as if a director of Shakespeare's Henry V had decided not to have a crowd of people listening to the king's famous rallying cry. Or as if a director of Hamlet had told his main actor not to bother appearing to be mad.

Plays and operas are often updated or set in eras and locations different from the ones imagined by their authors. And often a new and interesting light is shed on the inner meaning of those plays; Jonathan Miller's mafia setting of Rigoletto is a famous example. But it seems to me that a director who ignores the very nuts and bolts of the plot does so at his peril. You can add new layers of meaning, but you are not at liberty to take away the foundations of meaning.