This movie is part one of the story based on the last book in the Twilight series. Bella, a human, and Edward, a vampire, are to be married, which means at some point she must become a vampire. This is something Bella has wanted for a long time, so she is delighted.
Early in the movie, Edward tells Bella about his past when he decided to give in to his blood lust as a young vampire. However, he killed only murderers, as Bella kindly points out. Edward tries to get Bella to consider changing her mind about becoming a vampire but she is not to be persuaded.