In the film, Professor Moriarty is up to his usual nefarious "bad works" (in this case an attempt to start a small war to increase sales in for the arms firm he's bought into) and only Holmes and Watson can stop him.
It wasn't bad, but it just wasn't very exciting, and gave me the sense I'd seen it all before. I had the feeling that it was pretty much exactly the same as the first film (you could probably swap a number scene from one and put it in the other, without doing much harm to the plot), which made me feel "what's the point of going to see it?"