Parallel story is [Kain and Suu arc]  [3rd invasion] .
Just as Kain is running out of options, so is Juron. He, too, is running out of everything.
Don't know where to begin?
While ultimately it doesn't matter which arc you begin with, Cecil arc is recommended if you want my recommendation.
And then Masu arc + Shattered union arc.
Feel free to visit the lore section as well which holds stories of its own.