Fan Translation:
"The Holy Grail", which is said to appear once every few decades, grants its owner every single wishes.
Seven mages (Masters) form a contract with seven familiars (Servants), and kill each other until only one is left in the "Holy Grail War" to gain the Holy Grail.
The protagonist, Emiya Shirō, ends up getting involved in the "Holy Grail War" after accidentally forming a contract with the strongest Servant, Saber.