In a nutshell:
A depressed and edgy teen Albus goes on an adventure with his near-perfect friend Scorpius to save Cedric Diggory, after being urged to use a Time-Turner by the illegitimate daughter of Voldemort and Bellatrix. They manage to save Cedric Diggory but in the process they accidentally create an alternate universe where Voldy wins so they have to go back in time to reverse it and *insert 200 pages of time-travel*. They have a final showdown with the illegitimate daughter of Voldemort and Bellatrix and happily ever after.
