I just saw the movie and I did not understood anything

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Don't read any further unless you are prepared for spoilers. The only scene that takes place in the present is the scene in the rest room. The other stuff is either flashbacks of the growing friendship between Diana and Maureen, or Diana's imagined future flashing before her eyes just before she is shot. The flashbacks show viewers the process of Diana becoming less and less selfish; a maturing that makes it possible for her to sacrifice her life for her friend.

When Maureen and Diana are asked which one should be shot and Maureen volunteers, Diana imagines what her life will be like from the point of Maureen's sacrifice. This imagined life is the first part of the Uma Thurman as a grown-up Diana stuff. During this process Diana realizes that if she allows Maureen to die without offering to die herself, her future life will be full of guilty conscience issues. Because of that AND because she sincerely wants to protect Maureen, she offers to die instead of Maureen. The last part of the grown-up Diana stuff (when all that strange stuff starts happening with her husband and daughter) is most likely Diana attempting to finish imagining her future life; in the seconds after she is shot, as her actual life is ebbing away.