Since the events of Underworld: Evolution, things have changed quite a bit. Humanity has discovered the existence of the vampires and lycans, and is now attempting to eradicate them. Selene and Michael are captured and frozen, Han Solo-in-carbonite-style. Thirteen years later, Selene wakes up but Michael is missing. Selene then begins searching for Michael, uncovering a lycan conspiracy and slaughtering hundreds of people along the way.
The Underworld series has never really been all that great. At best it's been "okay." Its real strengths were its well-thought-out mythology and the star power of Kate Beckinsale; its weaknesses were its direction and lackluster scripts. Awakening doesn't get any better in any of those categories, but it does have enough straightforward action to more than make up for the lack of real emotional drama. All this movie really has going for it is a story that's just good enough to justify the action, which is really fun.
Another interesting note is the fact that the 3D version of the movie is actually done very differently from most 3D movies. While most 3D films merely give a bit of "depth" to the movie, allowing the audience to tell which objects are closer or farther away, Underworld turns the 3D depth up way more than most other films even dare. The result is something that actually feels less like a movie and more like just "being there." It's surprisingly enjoyable.
If you're an Underworld fan or someone who just likes the thought of spending two hours watching Kate Beckinsale in black leather shooting people, Awakening is worth watching.