If you want to take somebody to their first opera, this might not be the one to choose. The music is terrific, the acting brings the subtle and intense emotional story to life, but it’s long. Very long. And the plot stretches plausibility even for an opera-going audience.

Which is strange, because the human story is only too plausible. Idomeneo, king of Crete, is due back from the Trojan War. On Crete, his son Idamante, who we presume hasn’t seen his father for the ten years of the siege of Troy, awaits him in company with Trojan captives including the princess Ilya. She was rescued from a shipwreck by Idamante, and has fallen for him. Having lost her entire family and home city to the Greek army, you can imagine she feels quite torn about this, and is keeping it to herself, apart from a touching confession to the audience.

Idamante has fallen for her too, but can’t break through her reserve. Under Katie Mitchell’s direction, his vain attempts to woo her in a long aria, protesting ‘I am blameless, but you condemn me’, happen over a long dinner in which she won’t meet his eyes, can’t eat, and grips her wine glass as if it’s the only solid thing in her life.

Idomeneo is reported dead in a short, dry recitative with a bare harpsichord. And Idamante absorbs the news in silence, an element very under-used in opera. Then, broken, he runs out. The father he’s awaited for so long will never return.

Except that he has returned, washed up on a bleak beach that’s hemmed in by a massive stone wall, still in his army camouflage. And the first Cretan he meets is Idamante, walking alone in grief. But when Idomeneo learns this is his son, he recoils. ‘Don’t leave me!’ pleads his son, as Idomeneo runs off. ‘The father I longed for, I find him, then I lose him,’ he laments. Throughout the opera, this rejection torments Idamante. But Idomeneo has reason to wish he hadn’t met his son on the beach. In the fear of the storm, he promised Poseidon that he would sacrifice the first person he met on the beach – Idamante.

Back when it was written (first performance 1781) the traditional story of a father promising the gods a sacrifice, only to find the gods ask for their own child, would have been recognised from both classical and biblical traditions, and accepted as a fine plot for tragedy. Fate can be cruel, and tragic heroes must suffer.

What Mozart and his librettist brought to the story, and what Mitchell draws out with beautiful performances from the cast, is the sense that the gods are unjust and cruel to ask such a thing. The word ‘blameless’ comes around again and again. Even Poseidon’s own priest sings, ‘The son is not guilty, the vow is inhuman’. But to modern eyes, the idea that a panicked promise to the god of the sea would actually commit a father to murder his own child seems… melodramatic. And the fact this production is set in modern dress, apparently in a business hotel overlooking the sea, makes it all the harder to accept Idomeneo’s knowledge that his duty as king is incompatible with his feelings as father.

When a storm racks the ferry port, the frightened crowds huddled in the First Class lounge with the stewards, the wrath of Poseidon does become tangible. But there’s still a gulf between fear of natural disaster and willingness to murder your child in cold blood. The impossibility of finding a private moment in a hotel full of schmoozing couples and attentive flunkeys is recognisable. But the need to believe that a god would enforce such a terrible promise, made in extremis, is at odds with the all-too-familiar human stories of love unrecognised, unreturned, or simply unconfessed.

Mitchell eschews spectacular, supernatural visuals, apart from some lightning in the darkening sky over the sea. The orchestra has a free rein to bring the storms, both emotional and meteorological, to life. But in some ways this makes it harder to accept that the supernatural element is a metaphor, or a dramatic device, to open up the emotional realities. Is Idomeneo deluded, suffering PTSD from the long war? If he is, his comrades and citizens are colluding with him.

There’s a chilling horror to the bloody reality of human sacrifice. To the Wire generation, the carrying of plastic sheeting and other hardware across the beach, ready to receive the not-yet-dead body, is all too suggestive. And that gives the agony of Idamante’s duet with his father a depth and truthfulness that do justice to the music. You won’t leave the theatre thinking of human sacrifice as an abstract idea. It’s a long evening, but it resonates longer still.

Till 9 July 2010.