F/SN the visual novel has 3 routes.
There is one route where Shirou and Saber are the main characters. This is what Studio DEEN adapted to a TV show in 2006, albeit rather poorly.
The second route, Unlimited Blade Works is where Rin and Archer are the protagonists, this is what will be adapted this year.
The third route, Heaven's Feel, follows which is what the movie(s?) will be based on.
Bear in mind I have not actually played the original VN (should get to it at some point) but that's the general idea.