It picks off very shortly after the previous film: John Watson has moved out, but still hasn't quite married his fiance Mary, and Irene Adler is still working for Moriarty. Holmes is on the trail of the Professor, and believes him to be responsible for a series of bombings throughout Europe. After a fun action start, Watson picks up Holmes for his stag night, and things take a different turn when they bump into gypsy Madam Simza.
For me, Holmes's imagined walk-throughs of the fight sequences were one of the most memorable (and effective) parts of the first film, and Ritchie employs the same technique here, but also subverts it nicely in each case. The other best bit - the bromance between Holmes and Watson - is still there in all its glory, and slightly tweaked: this time around, Holmes doesn't seem to have the slightly spiteful edge that the first film inherited from its House heritage.
Jared Harris makes a nicely controlled Moriarty, there's (for me) a little too much Stephen Fry as Mycroft, since Fry is comic relief here, and not showing the levels of awareness that Holmes's brother should have. Noomi Rapace didn't really have much space to develop a character for Simza - I felt she was following along with Holmes for the purposes of the plot. As it happens, following Holmes was the best way to achieve her own goals, but I missed the exposition where she had to be convinced of that, at each stage. Maybe Simza's just much smarter than me. But perhaps I was distracted, because I kept trying to work out where I'd seen Rapace before. I finally figured out that she reminded me of Catherine Tate.
I wanted a lot more Irene Adler, as she was at the start of the first film, because she was great. At least here, Simza isn't a romantic character - thankfully, because Watson's just gotten married, and Holmes, even if he weren't obsessed with the case, is also emotionally involved with Adler, and shifting affections to Simza just wouldn't have made sense. I wonder how many arguments were had, over that, in Hollywood.
And a nice closing scene.