I'll keep this short; thanks to Greg for helping me to put this succinctly: Captivity is about a guy who drugs a girl's drink, imprisons and tortures her, then poses as a captive to have sex with her. That is the single twist and punchline of the film. It's torture as slow motion date rape. And, it's not even a good movie. It's not so bad it's good; it's just bad.
It should also be mentioned that among critics, there is a "spoiler code" that they dare not break, even though some were tempted to on this one because it is so vile. Why NO ONE had the cojones to step up and say, "this is garbage, and this is why," is beyond me.
Don't give your money to these poop-peddlers.