A little bit of overview for those who haven’t watched this series. Don’t worry, there’s no spoilers.
So first of all there is a war that is called the ‘Holy Grail War’. In this war 7 masters summon 7 servants fighting to win the ‘Holy Grail’, an omnipotent wish granting device. Each master is chosen by the grail to participate in the war and most of them have a wish they desire to be fulfilled. The only way they can win the grail is by surviving the war and being the last man standing. And so, 7 masters with their own servants fight to the death to win this grail.