A young man known only to the viewer as Kyon enters high school feeling somewhat disaffected. He talks about how much more fun life was when he still believed in Santa Claus.
As he narrates, Kyon meets Haruhi Suzumiya: a strong-willed young woman who introduces herself to her class as someone who has no interest in normal humans, only in aliens, time travelers, and espers. As the first weeks of class pass, he continues to notice her-- specifically her hair, which she wears differently every day (one day down, the next a pony tail, the next pigtails, then three ponytails, then four, etc). He mentions this to her only to receive a gruff response. The next day she comes to school with her hair cut short.
And so the two gradually develop a speaking relationship, mostly instigated by Kyon. When Haruhi complains that none of the school clubs are interesting (she tries each and every one, for one day only), he suggests that she make her own. So she does, demanding his assistance. She creates the SOS Brigade, devoted to finding paranormal people and phenomena. She takes over the Literature Club's room (and its only member, Yuki Nagato), and then forcibly enlists second-year student Mikuru Asahina, a "mascot character."
With no sure destination of what the club is about Haruhi, Kyon, Mikuru and Yuki start the SOS Brigade into nowhere in particular.