OK, here's the short version:
There is a magic war every 10 years where 7 mages get together and summon heroes from various legends to combat each other, and then the winner gets a wish from the holy grail. Or rather, that's what's SUPPOSED to happen. In actuality, the rules are fucked up beyond recognition, half the "heroes" are evil, and instead of granting wishes the grail just nukes everybody in a 10 mile radius when you touch it.
The main character is Shiro, an amateur mage who got rescued from a previous grail nuking and wants to become a hero. He accidentally summons King Arthur, as a girl, as his familiar for the grail war, and then they fight everyone else.
And I think I'm about out of words, since you only wanted 500, so that's as much summary as you're getting. Just stop being a lazy ass and read the Wikipedia article.