Albus Potter leads a normal life - or as normal a life as possible, considering that his father saved the world. But that changes very quickly when his sister Lily goes missing. When Albus and seven others are transported back in time to an ancient battlefield, they at first don't know what's going on. But they quickly realize two things:
This is the War of Hogwarts - they're 1,000 years in the past.
And in the immediate future, there is no way back to their time. They're stuck here until the battle is over. At the very least.