It’s a long way down to the stalls in the depths of Covent Garden’s newly refurbished Linbury Theatre, and further still (metaphorically speaking) into Niflheim, the kingdom of the dead ruled by Hel, the half-woman, half-corpse daughter of the god Loki and the giantess Angrboda. In Timothy Sheader’s staging of Gavin Higgins’ new opera The Monstrous Child, it’s a dark, well, hell-hole, where water drips constantly from above, where Hel spends her days scratching her name on the walls, exiled as she is for eternity with only the dead for company.

Turning this story into teen fiction, where a 21st-century teenage girl is made to self-identify with this weirdest of creatures, might seem like a big ask, and turning the resulting novel into an opera libretto (when you’ve not been an opera fan and have never written one before) might seem an even bigger one. But Francesca Simon is made of stern stuff and has pulled it off brilliantly, with the aid of an awesome performance from Marta Fontanals-Simmons in the title role. What Simon does so well is to engage the audience from the first moment and then set the pace at an ebb and flow that keeps you gripped throughout. The libretto is peppered with gags in up-to-the-minute language (“You’d think, after my brother the snake, they’d have stopped at one”, “My father shouldn’t have shagged a giant”, etc) but they’re never allowed to take over to the point that you lose the epic nature of the story that’s unfolding. And events occur fast enough to keep you interested, but without losing the idea that countless aeons are passing.

If only Gavin Higgins’ music had the same sense of ebb and flow. There is much to interest and even to love about it; Higgins certainly has an ear for a melodic line and the score is packed with compelling phrases, of which Jessica Cottis and the Aurora Orchestra made the most. But even when there’s a lovely melodic line, Higgins complements it with a hard-edged, dissonant backing: it’s as if you’re in the bit of the movie where the music is telling you that something really dramatic and scary is about to happen – but all the time. After 20 minutes or so, the constant musical tension just gets wearing, regardless of any of the score's other qualities.

Fontanals-Simmons anchors the whole thing with panache, actively involved in every minute of the opera, shifting seamlessly between sardonic wisecracks and true pathos, drawing you into empathy with her fantastic character. We are looking at a creature whose lower body has merged with a giant, fetid mound of rotting vegetation around which she drags herself, and yet Fontanals-Simmons gets us completely inside her head, sharing in Hel’s hopes, illusions, hatred, despair. It’s an excellent vocal performance also, strong through the range and proving that beauty of lyrical timbre needn’t prevent perfect diction with every word intelligible (no surtitles were provided, nor were they needed).

The supporting cast comprises both Royal Opera stalwarts and younger performers: Graeme Broadbent is a notable Odin, his character shifting between stentorian king of the gods and wheedling con-man in a way that will be familiar to lovers of Das Rheingold. Rosie Aldridge makes a telling contribution as Hel’s giant mother, as does Dan Shelvey with a bright tenor as the loved-by-everyone (well, nearly) god Baldr. Sets and costumes, by Paul Wills, are simple but highly effective. There’s much use of video projection and puppetry that’s very inventive and combines into a whole that’s far beyond the scope of what you’d expect on a smaller stage – the scene of Angrboda giving birth to her three monstrous children is marvellously gruesome black comedy.

How to end such a tale? I won’t spoil it, but suffice to say that Simon subverts any traditional operatic ending and provides something highly satisfactory to any opera fan who has asked the question “what happens next” at the end of a Ring cycle.

I’m going to assume that in staging this, The Royal Opera hoped to make opera relevant to a younger audience. I didn’t see many teenagers in the audience last night, and I have to question whether dozens will be flocking in through the rest of the run. But regardless of that and of my reservations about Higgins’ music, The Monstrous Child is a great show, inventive and superbly crafted. The new Linbury Theatre – for which this is the first opera – is a lovely space: comfortable, elegant and with good acoustics: this was a more than worthy inaugural operatic performance.