In this 4th game of the assassin's creed franchise, Desmond must relive the last significant memories of his ancestor, the Assassin Mentor Ezio Auditore, who sought out the secret library of the Mentor of the Levantine Assassins Altair Ibn-La'Ahad, in 1511. The game happens in Constantinople!
This comes straight after Brotherhood, it's still carries Ezio's story, except that in this one, you go back and visit some of Altair stories. I definitely recommend it, like in all the previous ones, in every new game, you get something new to do, which makes it exciting to play.