At this point in the episode, the twist comes in as the viewer is brought into CDC headquarters. As a sole scientist (presumably) is working on a cure, a la "I Am Legend." He's on the brink of giving up as his tests go wrong on him and he considers taking his life. Our survivors eventually make their way to CDC. When they arrive to CDC the entire area is in disarray. Bodies surround the building as they make their way to the front. Most of them quickly decide that no help is to be found here but Rick is adamant that someone is in the building and this is confirmed when Rick notices the camera on the outside of the building move, as the CDC scientist is watching them, hoping they'll go away. But his conscious gets the best of him as he opens the door for them and we are left with another great cliffhanger.
I was surprised by this departure but I feel like it's a minor thing to help set up the final episode for this season. The slower pace of this episode was bothersome at first, but all the quiet moments were given enough time to blossom into something more. With one episode left, I feel like it was way shorter than it should have been. As much as I've enjoyed the show, up until this point, I don't feel a connection to most of the characters aside from Rick, Shane and Andrea. And I feel this is due to season one's time constraints. It's been a solid season but it needs more characterization. I need and want to care.
Note: Some reviews have mentioned the fact that Daryl made no mention of his missing brother, Merle, which is clearly a misstep in character continuity. But I don't like Daryl or Merle enough to care.