Assassin's Creed II meshes an amazing plot and great mechanics for an excellent second entry in the franchise.
The story continues where Assassin's Creed 1 lets off, literally: only ten minutes passes in between the games' settings. ACII begins with actions that you don't quite understand, but soon blossom into understanding. I don't want to give anything away about the present-day Desmond plot, so suffice it to say that Desmond Miles winds up going back into the Animus- you'd better hope you enjoyed your brief time as Desmond, because your experiences with him, though enjoyable and eye-opening, are very few. Desmond hops into the body of his Renaissance period Italian ancestor Ezio Auditore da Firenze this time. Ezio is an engaging character from the start, showing all the personality a teenager can show- he is brash, romantic, and protective of his family and friends. An early traitorous act involving the Auditore family causes Ezio to be motivated by revenge, which grows into Ezio becoming an Assassin.
There is also a sub-plot in ACII called "The Truth". It is comprised of a series of hidden emblems which contain pieces of a video locked by some challenging puzzles. I cannot comment on the impact of "The Truth", as I have not yet collected the entire video.
Ubisoft Montreal made very clear that variety was the name of the game this time around. Mission structure is notable here- while the missions are more linear in nature, no two story missions are exactly the same, from flying Leonardo Da Vinci's flying machine to dodging hijackers while driving a carriage. Combat is also much more varied this time around- different guards attack in different ways and with different weapons, while you in turn can use the new monetary system to purchase better armor and weapons. Combat is still awkward, however: enemies will still wait their turn to attack you, can still usually be easily counter-killed, and will completely ignore you while they are fighting your allies, allowing you to simply walk up behind them and assassinate them with the hidden blades. Speaking of assassinations, new assassination techniques, while appreciated, blow my mind as to why they weren't included in the first game- should Ezio really have to "learn" to jump down and assassinate an enemy from above? Finally, the last notable gameplay tweak is in the inclusion of Assassin tombs, which allow for periods of fun puzzle platforming.
As for presentation, the sound and score are excellent. The graphics are nothing short of beautiful, aided by the perfect Renaissance Italy landscape. No detail is overlooked anywhere. Viewpoints in the game offer fantastic views of the huge, sprawling cities.
Assassin's Creed II is a triumph to all those who believe that a sequel should truly improve upon the successes and faults of its predecessors. With an ending that indicates a dark future for Desmond and a bright future for the franchise, we can only happily await the third installment in the battle of the Templars and the Assassin's.