This movie is not a stand alone piece, and not recommended for viewing if haven't already seen the entire "Suzumiya Haruhi no Yuutsu" TV series. You wouldn't understand why things happened the way it happens in this movie without this background information.
Kyon wakes up one morning and discovers that his world have changed literally overnight. He goes to school and there is no sign of existence of Haruhi. He suspects that Haruhi did something again that changed the world, but this time his hunch turns out to be wrong. He needs to find out who and how this change was brought.
Kyon thought he would have a more peaceful life without Haruhi, but he's in for a surprise when he discovers that he really isn't enjoying the "alternative" reality that excludes her existence from his life.
This is a well crafted plot where many of the past Haruhi sagas are woven into this entirely new story. It gives everyone an opportunity to see what the world would be like without Haruhi Suzumiya as we know her including Haruhi herself.
One thing I don't understand in this entire story is who shot the reprogramming needle towards the end of the story.
This is also one of the all time best anime movie to come from Japan. Definitely worth a watch if you've been following the Haruhi Suzumiya series.