alright,
there are essentially two stories throughout this game. they are both about the same thing and run parallel to each other, but in different time periods.
in the not so distant future (2012, actually), a young man named desmond, played by you, has been confined against his will be a company named abstergo and is forced to undergo research into his mind. the lead researcher explains to you that insticnt is nothing more than genetic memory, and that all of anyones forebearers memories are locked inside of an individuals dna. this research is being done on desmond through a device known as "the animus", and essentially allows desmond to play back through the memories of one of his ancestors named "altair" who was an assassin whos life played out during the crusades, 1000 years prior. through a series of investigations and assassinations, altair (desmond) is drawn through a complex story of faith and knowledge in order to find what abstergo has placed so much importance in, the piece of eden. crusaders, templars, saracens, turks, the brotherhood of assassins, and even king richard the lionheart, all players in a story that encompasses Masyaf, Acre, Damascas, Jerusalem and the land in between, or The Kingdom. journey through the past, to discover the future.
if i were to rate it, i think id have to give it 9.5 out of 10. absolutely beautiful graphics, a story that is enough to drive a conspiracy theorist wacko, a free running protagonist that makes spiderman look like a kid, and unique combat, that at least a dozen times has made me and my friends go, "OH SNAP!"
the downside is that, like the man said, it CAN be a bit repetitive, and unless you are into the story, one pickpocketing feels very much like any other. the assassinations make up for it though, running through a crowd of people after just cutting down a merchat king while being chased by literally dozens of guards just isnt getting old for me.
seriously, if you can, check it out. and if you are struggling, play at least until memory block 4. it starts getting very interesting about there.