There are two endings in the game. Unlocking both endings is required to get the "End Of Night" and "Just A Memory Now" achievements. After defeating the huge mutant Boss, you must decide whether to cure Mia or Zoe. There is only enough serum to cure one of them. Curing Mia unlocks the first (good) ending and curing Zoe unlocks the second (bad) ending. After making that choice, it is still quite a while before getting the ending. These events occur at roughly 66% story progress.