The story is that Halvard Solness (Ralph Fiennes), a naturally talented but untrained architect, has become a great success but is worried that his days as the master builder may be coming to an end. He fears that younger, more talented architects will come along, like Ragnar (Martin Hutson) who works in his office. A young woman Hilde Wangel (Sarah Snook) arrives one day claiming to have met Halvard 10 years previously, when he made advances on her together with some outlandish promises (think trolls and castles in the sky).
But that is part of the play that I didn't like so much. I couldn't make out if Hilde was delusional or calculated and I'm not sure if she is meant to be either (maybe it is David Hare's adapatation?). I think the trailer interview with Ralph Fiennes talking about the play being a psychological thriller doesn't help. Reading up on the play afterwards Halvard has been described as a middle-aged man showing off in front of a young woman and that I get. But, in his work practices Halvard doesn't so much flirt with youth as block it, he manipulates his young book-keeper Kaja's (Charlie Cameron) feelings for him in order to keep her fiance Ragnar from striking out on his own.
Hilde brings with her a slightly fantastical element, perhaps she represents a younger, freer, bolder Halvard before life experience and tragedy shaped him? You can't really take her at face value because she is quite fanciful which is why I question whether she is delusional. But then Halvard's wife and his doctor friend fear he might have his own mental issues.