With the apartment block under quarantine a doctor and a SWAT team enter to try and neutralize the threat. With news that what ever the infection is being transferred by blood being good news they still need to find all those who were infected in order to cure them. Meanwhile another group are making their way into the building via the sewers to try and rescue their friends
There's been a few times that I have watched a sequel before I've watched the original and I actually believe that whilst a good sequel delivers a continuation of a story it also manages to work as a standalone movie for those who didn't watch the first movie. And I have to say that "[Rec] 2" achieves that because you have the continuation of the situation in the apartment block and then for those who don't know the back story you have this movie featuring a SWAT team entertain the building.
But whilst "[Rec] 2" achieves being watchable as a standalone movie I can't say it did a lot for me and the moments of horror shot via the cameras which the SWAT team wear were weak. But the visual nature of the horror is meant to combine with the claustrophobic nature of the building but that didn't work for me either and far too often "[Rec] 2" edged close to feeling like I was watching someone else playing a shoot em up. This is contributed to by the lack of character depth and so you have no interest in who lives and who dies.
What this all boils down to is that "[Rec] 2" came up short for me as whilst it appears to work even if you haven't watched the first movie it fails to work as a horror and often comes across like a shoot em up rather that a horror movie.