Archer explains to Shirou that there's two of that pendant that Rin saved his live with, because one of them came from Archer. Since she didn't have that catalyst when she summoned him, she assumed she'd just summon a servant at random. but no servant is summoned by chance... the summoned heroic spirit, not the summoner, possessed the catalyst. The concept of time does not exist where the heroic spirits are summoned... yada yada he explains the obvious by now in case anyone still didn't get it.
Lancer says he'll take care of saving Rin from Shinji... saving Rin was what he was ordered to do anyway.
Saber still doesn't fully trust Lancer, but she lets him take care of her master so she can stay here with Shirou...
Lancer comes to save her... but then Kirei suddenly comes in to stop him.
meanwhile, Archer is talking to Shirou and Saber about why he wants to kill his past self... his pact to become a guardian made him see an endless existence of nothing but killing people in order to save thousands for each 1 person killed.
"I just didn't want anyone to cry in the world I knew..."
now that he's fully explained to himself.. he hands shirou a sword and tells him to do what's right and kill himself without becoming a heroic spirit.
oh I see.. Kirei is there because Lancer is ignoring his real orders of killing Archer.
Lancer: "There's no need, he'll do himself in without my help."
Rin learns from Kirei that he's the one who killed her father, and all this time he's only been protecting her to use her as a pawn.
Kirei orders Lancer to kill himself with a command spell.. and says he needs Rin to act as the holy grail.
Shirou says that he and Archer are two different people, and that he'll never regret living by his ideals.
"If you're the manifestation of my ideal, then I'll just have to get rid of that mistaken ideal myself...
I won't become you, that day will never come."