So as to not bore you, I'll give you the abridged explanation.

A bunch of greedy humans who happens to be able to use magic summon forth and contract with Heroic Spirits of the past, such as myself. They then proceed to fight each other in hopes of obtaining the legendary wish granting Grail. Of course, I'm sure you can already guess that it isn't that grail. And I'm also sure you can guess the actual outcome of such a promising item.