When science fiction decides to get all deep and philosophical, it always comes down to questions of free will. Do we choose our actions, or are they already totally predictable to someone who could glimpse the future? For example, Terminator 3 caused a lot of controversy with an ending that suggests John Connor can't escape his destiny as a post-apocalyptic leader. Are we just puppets of a future history engraved in stone? We settle these debates once and for all, and list the four different types of fate-vs-free-will stories in SF, below.
The whole idea that our actions are determined ahead of time is more metaphysical than scientific, although some have claimed that quantum mechanics proves our decisions have already been made. But the idea of "fate," or "unshakeable prophecies," really belongs more in the realm of mythology and gods than in a story about a rational, observable universe. As soon as you start talking about someone being unable to escape his/her destiny, suddenly there's a guy with a white beard talking for like ten hours. Like this guy: