Fate/stay night - Season 1 Episode 9 Elegance in the Moonlight

Saber is eager to go to the temple but Shirō has insisted that they won't be going because it seems too dangerous. Nonetheless, as soon as Shirō falls asleep, Saber heads off by herself to the temple, not only to be confronted by the servant identified as Assassin. Unlike any other servant, Assassin actually reveals his true identity to Saber, as Sasaki Kojirou. Although the talking ends here and the fighting begins but while this is going on Rider sneaks past them and enters the temple to fight some woman. Who is this mysterious woman and was Shinji planning this all along?