Up until the final "revelations" the audience saw nothing more than improbably tough, brave crewmen desperately trying to sacrifice themselves for a war the audience knows nothing about, other than that the unseen lizard monsters are winning and the four characters seem to really hate that.
So, after a half hour of pushing levers and control rods and crawling through a radiation-filled spaceship, the writers spring their two surprises on the audience. First, one of the characters isn't who he said he was, and, second, the other characters weren't where they thought they were. Oops!
I would have felt cheated, but, frankly, the characters weren't very believable in the first place, and their situation never seemed overly compelling because no one bothered to set it up for the audience. There were several interesting tid-bits thrown in the mix, but the filmmakers just treated them as window dressing and concentrated, instead, on the least interesting, and most clichéd, aspects of the story. Oh, well.