***********SPOILER ALERT***************
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The UFO lands in Manhattan along with smaller UFO's acting like a Noah's Arc attracting all the species because we're all going to be destroyed to ensure the longevity of the earth.
Keanu Reeve's plays the alien Klaatu who takes human form. He is shot in the begining of the movie and later interogated by the United States government. He later escapes with the help of Jennifer Connely and tries to make his way back to his ship.
While in the presence of Jennifer Connely and her step son (Will Smith's Real Son), Klaatu decides not to destroy the human. He is persuaded by her friend that humans can change. There was no big event that made him change his mind he just looked at Jennifer Connely hugging her step son when death was imminent and he says "I SEE A DIFFERENT SIDE OF YOU!!"
His robot protector Gort transforms into all these little nanonites destroying everything man-made in his path and later Keanu disables it and saves the planet the end.