Since you're asking for the most "natural", "end up" is not necessary. However, your first question would feel more natural to me if it were worded thus: "How will ( or How does) this movie end?"
Hi, How does this movie end? (Not end up). I cannot tell you how our relationship ended. (Not ended up).
End means finish. End up is an idiom, which is used when you find yourself in a place or situation in which you did not expect to be, e.g. She ended up in the hospital; he ended up doing that difficult task by himself, etc.