The basic plot set-up: The Imperial army is invading Gallia, the nation of the protagonists. Three kids from a small town--Alicia, a member of the town guard; Welkin, the son of a war hero; and Isara, Welkin's sister--become embroiled in the war effort when the Imperials attack them.
Oh, and sometime later there's a big-breasted, silver-haired stoic who wields a huge lance that shoots blue energy beams. It's pretty awesome. The first episode has been fansubbed, so you should watch it. Because if you don't...
Alicia will want a word with you.