In the Assassin's Creed series you are actually playing a 20th century man called Desmond. In AC1 he is captured by the Abstergo company and put into a machine called the Animus, a machine that regresses Desmond through his "Genetic memories". In AC1 the ancestor whose memories you "relive" is called Altaïr Ibn-La'Ahad, a member of the Assassin's guild in the 11th and 12th centuries. The Assassin's sworn enemies are the Templars who you find out are still about now and they are the Abstergo company. In AC2 you continue to use an Animus but with some 20th century assassins who helped you to escape Abstergo. In AC2's animus you go back to a different ancestor called Ezio Auditore, whose story you continue in AC:B.
Ok so now you know the outline of the story but it gets much more in-depth than that. So the memories you are reliving, the main thing goes on is that the Templars are trying to use alien artifacts to control people and bring about world peace and basically help people. The Assassins hate this because, although they also are for the good and so on, they hate the mind control that the Templars use. Assassin's Creed is choc full of moral questions about what is wrong and right and if either group is really "the good guys". Of course, while you are musing that over, you are probably wading through rivers of city guard's blood without a qualm.
As in the previous AC's, your main weapons are your hidden blade and your sword, and you can still execute two enemies at once with the twin hidden blade added in AC2. AC:B added a few new weapons to the arsenal you have in AC2 though, namely a crossbow, usefully enabling long distance stealth kills, a few other heavy weapons likes maces and spears, and poison darts (a weapon I didn't realise was in the game and only found that out when getting the pictures for this review!).
The far weightier thing added into AC:B is the Execution Chain. Basically when you execute an enemy in combat you can immediately select another enemy and execute them and so on, only stopping if you get hit or if you run out of enemies!
In AC:B they have increased the revenue source from AC2. In AC2 you have a villa called Monteriggioni which you upgraded and it regularly deposits money into your chest. In AC:B you upgrade the whole of Rome, as the game is only set in the one massive city, from little tunnels to enable fast travel about right upto the Colosseum itself!
Then there are also the guilds. In the city there is a Courtesans guild, Warriors guild, Thieves guild and an Assassin's Guild setup by Ezio himself. Each guild is again something you can build up, by buying buildings and completing sub-tasks, which enhance both the groups themselves and give you little added passive bonuses and unique weapons. The Assassins guild however is actually a new feature added in AC:B whereby, at the touch of a button, you call a group of your Assassins to help you. There are three levels of the aid, the first two summon assassins to kill your target and then attack nearby enemies whereas having the third level unleashes an arrow storm that kills every enemy on the screen!
This is one of the largest areas that AC:B adds to the AC series. There are a growing number of gametypes, but the three main ones are Wanted, where you are on your own hunting one target and another player is hunting you, Alliance, which is the same but you are in teams of two hunting and being hunter, and finally my favourite Manhunt where you are in two teams of five, one team hunting and the other team hiding and only able to stun and then you swap. The multiplayer is full of challenges to complete and characters, abilities and weapons to unlock. Lots of fun and adds an interesting new dynamic to the series.
Though the plot of the AC series is quite complex and this game does add storyline that section of AC:B feels a little like a big DLC for AC2, however with the addition of the Execution Chain, the Assassin's Guild and the multiplayer this game should find itself a proud member of the series. Now go play it before AC:Revolution comes out!