The game consists of six different memory sequences, each of which consists of a few memories. You get achievements for complete each memory sequence and the present day missions in between them. There are also a total of 3 glitched memories. After you complete Sequence 6 Memory 5, Non Nobis Domine, you'll have to do the last present day mission, which involves rebooting a server at the server floor and uploading a movie/compilation of moments from the adventure. After this mission, you have to play the final glitched memory. A short credits section plays after this. You can't skip this. It's very short, only lasting a few minutes.
After all of this, a final cutscene will play in which you are congratulated for your actions by the Abstergo Entertainment leaders/members. You're given the choice of joining the Templar Order or dying. Then, the final credits begin to play. Sometimes people just quit out of these credits, which might not get them the achievement. You can actually skip these credits by holding
. Then you'll be taken back to regular gameplay. During this transition/ending, you'll get the achievement.