There is a war going on between masters and servants in order to attain the Holy Grail. Each master can call up one servant each, and their task is to eradicate the other servants, either by defeating them or killing their master. When there is only one master or servant left, he or she is granted the Holy Grail, and any wish they desire will come true.
Anime doesn't have to be repetitive and derivative, but the sad fact is it often is. Case in point: some of the arcs in the second half of Fate/Stay Night, which with just a few changes of mythology, verbiage and character names could have been lifted straight out of any number of other anime like, say, Shakugan no Shana (and, no, that isn't a joke or typo). As Episode 13 (the first episode in this set) kicks off, we have Saber suffering from the after effects of a vicious battle, and there's ample evidence that she's about to disappear into the ether due to her mana (which might be thought of as an analogue to a Flame Haze's torch in the other series) having been depleted. Sound at least a little familiar? Fate/Stay Night is a middling anime that has some good elements, but which often seems kind of warmed over offerings that many anime fans will swear they've seen before (often, in fact) in other shows.