For those who don't remember, Time-Turners were first introduced in Harry Potter And The Prisoner Of Azkaban, in which Hermione is secretly issued one so she can go back in time to take extra classes ...
... which in retrospect seems like an insanely mundane reason to time travel. It's like if Doc Brown used the DeLorean to go back and tape an episode of Miami Vice he missed. But at least that Time-Turner was only able to go back a few hours or so. That's not the case in the new story.
The play finds Harry Potter's son Albus and Draco Malfoy's son (and presumptive future Bond villain) Scorpius using an illegal Time-Turner to go back decades. Not surprisingly, their incessant time-meddling changes the present, resulting in a depressing reality wherein Voldemort is alive and Harry is dead.
So the whole thing is basically a rehash of Back To The Future Part II, stopping short of making Hogwarts a trashy Casino Hotel and giving Professor McGonagall ludicrously cartoonish breasts.
Manuel Harlan/Palace Theatre
"Expecto Pa-- wait, that's just my finger."