More style than substance, and more like a video game than a coherent story, Carter is basically a long frenzy of fighting with all kinds of weapons and extended vehicle chases. It tries to be shot in "real time" and all in one take, which just adds to the confusion. It doesn't make much sense, but if you want to see exploding molars and digital pigs, this movie has both of those.
The "story," such as it is, involves a zombie virus that emerged from the Korean Demilitarized Zone. At various points, the CIA, South Koreans, and North Koreans are all trying to capture the hero, who has no memory of who he is or what's going on, with nothing but an ear piece in his ear with a woman's voice telling him where to go if he wants to live and the body memory required to commit mass slaughter with knives, hatchets, guns, etc. The story is almost irrelevant, a break between fight scenes. It isn't so much exciting as it is headache-inducing, and by the time we learn what's really going on, the movie has gone on for way too long and the viewer is too exhausted to care.