In an alternate version of a familiar world, Yuki Nagato, a quiet girl who loves reading, runs North High School's Literary Club. The club's only members are the overly cautious Ryouko Asakura and the ordinary Kyon. Yuki deeply treasures the calm times they spend together, especially with Kyon, toward whom she has developed romantic feelings. The group is occasionally joined by the contrasting personalities of Tsuruya and Mikuru Asahina, who visit regularly.
One cold December evening while walking home, Yuki is approached by a girl named Haruhi Suzumiya, a peculiar student from the prestigious Kouyouen Academy. Haruhi, searching for aliens, time travelers, and psychics, asks Yuki to draw strange symbols on the ground, claiming it's part of capturing Santa Claus. Later, Haruhi and her companion, Itsuki Koizumi, unexpectedly join the Literary Club, with Haruhi taking on the role of executive president.
As a result, the once-peaceful Literary Club begins a series of chaotic adventures orchestrated by Haruhi, bringing Yuki and Kyon even closer together.