It's called the Holy Grail War. Magi summon heroes of a past age into one of seven Servant classes--Saber, Lancer, Archer, Rider, Caster, Assassin, and Berserker. I was called into the class of Lancer, and my Master's name was Kayneth Archibald El-Melloi.

The point of the war is for the seven Servants to fight until only one remains. The winning Master and Servant pair then claim the Holy Grail, a relic which is able to grant any wish. Even if that wish should require an impossible miracle.

As for my experience in particular...my Master never trusted me. He knew the kind of things that had transpired in my lifetime, and though I did all I could to prove otherwise, Kayneth believed I would steal his fiancee and run. But I wouldn't have, I...

...I had no wish for the Grail. All I wanted was to win for my Master as his loyal knight, and...and bring him victory as I failed you in life.