One thing I didn’t mention - everyone in the world is born at age 10, trained to fight, and either die on the battlefield or when they hit their 10th year (20 years old) in a process called “homecoming”. The main goal of the protagonists is to live beyond their artificially short lifespan, and one of the two protagonists (an Agnian girl, Mio) has only three months left.
There’s a lot more to it than just that, but spoilers. This is just the general gist as shown in the reveal trailer.