So the timeline starts off a year after the war. So, the year is 1999. Harry and crew are not at Hogwarts.
The storyline is this, Voldemort before he died made a new Horcrux. So, he comes back and it takes him a year to gain his strength back. No one knows he is back until he is healed up and makes his presence known by very few Death Eaters alive and in hiding. Then they work on a plan to take over the Wizarding World. This time Voldemort's new Horcrux is another person but no one knows and this time the person doesn't have dreams or can talk to snakes.
Also, Voldemort returning causes different magical worlds to collide.