LONDON • When we last left Harry Potter, in the final pages of J.K. Rowling's Harry Potter And The Deathly Hallows, he was a middle-aged wizarding bureaucrat sending his second child, Albus Severus, off to Hogwarts for the first time. His life, marked in part by violence, danger and grief, seemed to have settled down, and those of us who had worried about him through so many pages for so many years felt relieved at his apparently happy ending.

Now Harry is back - in a play, this time - and we have to reconsider the whole thing. All is no longer well in Potter-world. But though it is full of new difficulties for its famous protagonist, Harry Potter And The Cursed Child, which had its first preview in the West End in London yesterday, felt like an author sending a Valentine to her long-time fans.

Here onstage were all the characters we had come to know so intimately: Harry, now married to Ginny Weasley; Hermione and Ron, now married to each other and behaving, at least until Rowling worked her mischief, in their old familiar ways.

For those of us in the audience, it was jarring to see the reanimation of characters we thought had been put to rest, who in our minds had been suspended forever in time and place.

We had to readjust our sense of their reality, and replace what had been in our imagination with what the author put in front of us.

Cursed Child, with a story by Rowling, John Tiffany and Jack Thorne, has been years in the making, and the subject of keen anticipation - even though relatively few people will ever get to see it, compared with the many millions who have read the Harry Potter books or seen the movies.

It was odd, too, to watch the action unfold onstage. Reading the Potter books was for so long a rite of passage for so many children and the movies always felt like faithful adaptations that did not advance the plot.

But this was something completely different, this sensation of coming to the story not alone in the corner with a book, but in a large theatre in the company of about 1,400 other people.

Who knows what adjustments the play will go through before it opens; previews are often a time for tweaking. I began the evening slightly worried about what Rowling could do to a narrative I had read in its entirety years ago and, like many fans, had always associated with a particular time in my life.

But above all, she is a storyteller, and once more, I could not resist the ingenuity of her plot and her joy in her fictional world.

Here was a host of memorable characters, many of them making what amount to quick cameo appearances, much as a star might drop into a movie for a few minutes. Here was a second generation of new characters, including Scorpius, the unexpectedly delightful son of the decidedly undelightful Draco Malfoy, and, of course, the troubled Albus, whose adolescent struggles to make sense of himself, his friends and his family form the focus of the play.

How exciting, when you have just re-read Deathly Hallows and been reminded of what happened, to see what Rowling does: She thrusts us back into that concluding scene, making it the first scene of the play and putting us on Platform 93/4 as the characters wait for the Hogwarts Express.

She then rushes rapidly forward a few years and shoots off into all sorts of unexpected directions. And then we get to learn the thing that most drives every fan in these situations: What happens next?

Exciting scenes feature, among others, the Sorting Hat (wittily staged); the grumpiest of the centaurs, who is extra grumpy this time around; the evil Dursleys; and one of the book's more diabolically unpleasant characters, who turns up at the end and whom we had hoped we had seen the last of.

The tantalising question arises of what might or might not happen to the present if you try to tamper with the past. Albus Dumbledore himself appears, in a painting that speaks to us, as paintings do in Potter-world, followed by as good an explanation as you will ever hear of why we should not mistake a portrait for an actual person.

There is magic, right onstage, including some books that fly from a bookshelf and speak to us; a dexterous use of fireplaces as pieces of the wizarding transportation network; and an amusing wand fight in which a pair of wizards fling curses at each other. There are some creepy dementors that waft out over the audience, minus, at least from the upper seats, the cold draft that generally accompanies them.

"It was even better than I thought it would be," said Ms Iman Khabl, 23, who had come from Edinburgh, Scotland, to see the play and who said she had been thrilled to find references to sometimes abstruse details from all seven books. "It was such a nice nudge to the fans, that they brought everyone back, that they kept our favourite characters while bringing in so many other things."

The play attracted the usual complement of superfans, including people dressed in Hogwarts robes bought from the Internet; people who had flown from California the day before and were being kept awake by little more than caffeine and hysteria; and a pair of newlyweds who ditched their honeymoon in Iceland for the chance to see the play.

Rowling has done an excellent job of shrouding the production in suspense, letting out the occasional detail - Hermione is played by Noma Dumezweni, who is black; Albus is, basically, a disaffected youth - when it suits her. It is hard to believe that further details will remain secret until opening night on July 30. The play's script, No. 1 on the Amazon pre-order list, is to be released on July 31.

But for now, even fans lucky enough to procure tickets do not know how Cursed Child ends. The play is in two parts. Only the first half was presented yesterday, and it ended on a truly shocking note of suspense. We would not learn what happens until the first preview of the second part, on Thursday night.

One thing we still are not sure of: Which character is, in fact, the cursed child?

NEW YORK TIMES