In an alternate version of a well-known reality, Yuki Nagata, a quiet girl who loves reading and gaming, serves as the president of North High School's Literary Club. Her only fellow members are the overly cautious Ryouko Asakura and the completely ordinary Kyon. Yuki cherishes the calm, peaceful moments they share, especially those spent with Kyon, for whom she harbors romantic feelings. Their circle expands with the frequent visits of Tsuruya and Mikuru Asahina, two friends with contrasting personalities who often drop by the club.
One chilly December day, while walking back to her apartment, Yuki is persuaded by a girl claiming to be trying to capture Santa Claus to draw strange symbols on the ground. The girl introduces herself as Haruhi Suzumiya, an eccentric student from the prestigious Kouyouen Academy who is searching for aliens, time travelers, and espers. When Haruhi and her companion Itsuki Koizumi suddenly appear in the clubroom, they declare themselves official members, with Haruhi installing herself as the executive president.
With the Literary Club's tranquil days now a thing of the past, Yuki and her friends are swept into a series of misadventures under Haruhi's leadership—each event drawing Yuki and Kyon closer together.