Leos Janacek disliked long operas, and the first act of The Makropulos Affair is a masterclass in how to set up a drama without an ounce of fat. There’s a prelude: driving motor-rhythms, surges of emotion, and somewhere in the distance – far away (or long ago) – the sound of trumpets. The curtain rises and we’re tipped brusquely into a lawyers’ office in the early 20th century. The lawsuit they’re discussing is long-winded and complex: aren’t they always? No matter. By the end of the act, these blustering professional men have been interrupted by the magnetic and imperious diva Emilia Marty, who knows things about the century-old case of Gregor vs Prus that no living person could possibly know. We’re intrigued. End of Act One. That’s not hard – is it?

Welsh National Opera clearly disagrees, because at that point the tenor Mark Le Brocq (playing the solicitor Vitek) walked out with a flipchart, broke character, and after telling us that the first act was incomprehensible even to the cast, proceeded to recap the whole plot so far, as if the lawsuit were anything more than a MacGuffin. That was embarrassing enough, and you cringed in sympathy with Le Brocq (imagine being delegated to deliver such an admission of artistic defeat). He finished with a spoiler: the big reveal from the very end of the opera, completely blown.

Why? Why do opera professionals do things like this? True, opera has long been the subject of abuse, some of it partially valid, but much of it pure prejudice. And the most vacuous libel of the lot is that operatic plots are unusually absurd or complicated. Bollocks: any good mystery begins with a puzzle, and large-scale narratives in any medium become confusing when reduced to a dustjacket summary. Try retelling Game of Thrones – or pretty much anything by Shakespeare – in 100 words. But only opera routinely apologises, begs to explain, and – in a tacit admission that any aspiration to function as entertainment has long since been abandoned – publishes synopses that reveal vital plot twists. Why this cringe? Why this lack of faith? Why break a brilliantly cast spell, and run up a wholly unnecessary white flag?

The odd thing is that the director of this new production, Olivia Fuchs, is superb in Janacek. Her Katya Kabanova at Holland Park and Cunning Little Vixen at Longborough were models of inventive, emotionally engaged storytelling. And so was this: Nicola Turner’s designs placed the drama in its proper setting, 1920s Prague, with back-projections and a couple of wondrous, unexpected moments of stage magic (dusty stacks of legal papers suddenly rocketed up to the sky) to suggest that there are more things in heaven and Earth. Fuchs used the bustle and chatter of all that Act One exposition to help draw the characters. Nicky Spence was an awkward, over-compensating Albert Gregor (the tightness and focus of his voice was a real asset here), Harriet Eyley’s demure Krista wilted like a cut flower before Marty’s icy charisma, David Stout sang with a dignified swagger as Baron Prus, and as Marty herself, Angeles Blancas Gulin was simultaneously sensual and aloof, outlining her high phrases in flickering neon.

So again – why the failure of nerve? My guess is that Fuchs had her arm twisted, but in any case, having been jolted out of the opera’s world, I’m not sure I ever made it wholly back in. It might be unfair to say that the two subsequent acts (in which Gulin wore a crimson hairpiece and a white fright-wig before ending up completely bald) veered towards the camp, and that the brightly lit denouement felt too jumpy to allow the emotion to find its own level, even though Gulin – supported by bristling, fiercely unsentimental playing from the WNO orchestra under Tomas Hanus – absolutely blazed. Earlier, Alan Oke had somehow managed to be both funny and intensely touching as the decrepit Count Hauk-Sendorf. Overall, though, so much of Fuchs’s production felt, looked and sounded right that I wish I’d been able to experience it without a disruption that had a wrecking effect out of all proportion to its brief length and good intentions.

In London, Santtu-Matias Rouvali conducted the Philharmonia in a satisfyingly high-octane season opener. Vikingur Olafsson played John Adams’s recent piano concerto Must the Devil Have All the Best Tunes? – a one-note work from which Olafsson managed to draw a rainbow of textural nuance, before upstaging the whole thing with a tiny Rameau encore that might have been moulded from freshly fallen snow. And then Rouvali conducted Mahler’s Fifth Symphony: an insouciant, interventionist reading, constantly teasing and petting at individual phrases, to which the Philharmonia – from the low brass upwards – responded as if it were a gigantic string quartet. The players sounded smitten with their young principal conductor; and I understand that they actually are.