Falls flat on logic after a promising start

Rating: 5/10 (16 December 2013)

The screenwriters started with an interesting premise: "How about a man who thinks he is fighting against aliens for humans, but he's really fighting against humans for aliens?" The theme is accompanied by a suitably stark setting, with remnants of NYC sticking from a war-devastated ground.
By the third act, the screenwriters have written themselves into a corner, and the plot falls apart. Let's see, if I am a human who KNOWS that the man fighting me is being deceived by an alien into thinking I am alien, I'll wear alien masks and distort my voice while fighting him, to make sure he thinks I AM an alien. If I am an alien who has been monitoring an erratically behaving human servant, of whom I have 1000s of clones in reserve, I'll fly him into my inner sanctum without checking his cargo. Etc. Such glaring logical flaws are easily forgiven in a low-brow action flick, but not in an sf film that has higher aspirations.
I watched Toy Story 3 right after this upon recommendation of my son. What a paean to plot logic and character development by comparison. Similar concept: a hero thrust into a strange environment realizes that the seemingly good are in reality bad. Just better written.