Sequence 01, Memory 01 -- The Hangman
The CS for this mission starts out with Ezio sailing and climbing and traveling on his way to find a Library his father told him about -- but as he arrives he is attacked by Templars and you get to see him take out most of an army, before he is taken captive after seeing what looked like a younger version of himself?
Escaping the Hangman
As he walks out onto the platform and the noose is placed on his neck you see that he is old now, and you see him from the very first game -- assuming you played it -- if not, yeah, that Ezio in white on the other platform is a scene from the start of the first game.
Ezio manages to pull off a brilliant escape, killing the Templar Officer that was going to kill him with the same rope, and you land on the deck below, and gain control of Ezio.