Albus Severus Potter is sorted into Slytherin, and his only friend turns out to be Scorpius Malfoy, whose paternity is rumored to be Voldemort himself. Add to that the estrangement between Harry and his son, another uprising of dark powers, the emergence of a Time-Turner, Amos Diggory turning up at Harry's doorstep with a desperate request, and Harry's scar burning again which leads to ill-spoken words during a quarrel and hasty decisions - and perhaps the downfall of the wizarding world.
First of all, this story is told in script-form which takes away quite a bit regarding the inner motivation of characters. And this is perhaps the greatest flaw: I didn't quite understand why Albus would attempt to restore Cedric back to life? Granted, he's unhappy, feels misunderstood and unloved by his father, but change history, probably even erasing himself from history?
And let's not mention all the other head-scratch moments: Amos Diggory would come to ask for Cedric's return 22 years after he'd died? Grief can do strange things, I'll grant you that. But why doesn't anyone question his motives, and especially the strange niece no one has seen before? In one of the changed timelines Albus and Scorpius humiliated Cedric during the 2nd task of the Triwizard Tournament which turns Cedric towards the Death Eaters... Really? He had a lot going for him, he was head boy, had tons of friends, and all this wouldn't count for anything because he was humiliated during the TriWi-Tournament? We're not talking about Harry, Albus or Draco here, after all, we're talking about a boy who had everything going for him, loving parents, adoring friends. I don't buy that. (And let's not forget that when the boys try to correct their interference with the past, it's never told that they also correct that mistake, just somehow they find themselves back in the lake.)
Essentially, this is the story of parents and children: parents who lost their children, parents who can't connect with their children (and vice versa), and children who lost their parents, all this covered in prophecies and ridiculous time-travel. Had this been a novel I'd have expected more focus on the emotions, the relationships - and maybe then, the story would have worked better and the existence of Delphi would have been better explained: no one knew about the lovechild of Bellatrix and Voldemort?
But as is, the most relatable characters are Draco and Scorpius with a little helping of a Snape-cameo. The others are mere copies of their younger selves (especially when in alternate timelines Ron and Hermione restart their will they-won't they-routine). I liked the epilogue of Deathly Hallows, and I thought back then that Harry showed hard-earned maturity in advising Albus essentially to be who he is. Unfortunately he loses that maturity here altogether. Of course, all ends well, harsh words are forgiven and bridges built.
But while it was good to have another glimpse into the Potter-verse, I'm also somehow disgusted at such a blatant attempt at milking the cash cow just a bit further. Because let's be honest, The Cursed Child lacks detail, it lacks coherence and characterization. Quite frankly, JKR should have stopped while she was ahead instead of being lured in again into the spotlight and the call of fame and money.