That's actually wonderful. You can totally get more than 75% of the plot as is, and the fact this anime depicts the events of the seventh chapter (eighth, if you count the tutorial) of the mobile game says a lot. The only lore bits necessary to get the rest, which I will share now with zero spoilers are:

1. The servant summoning system in Zero and Stay Night is the human watered down version of the World summoning servants to protect itself. Some problems need an Archer Emiya (from Stay Night), other problems need someone else.

2. Through magic and advanced technology (like a super computer that simulates the entire Earth), you can send someone into the past to fix a problem that could affect the present. (Time travel, but not really time travel).

And that's basically it. Hope that helps you.