After the busy island adventure with the SOS Brigade and a family trip to visit relatives in the countryside, Kyon is just settling in to relax for the final two weeks of summer vacation. Unfortunately for him, Haruhi wants to get everything she can out of the precious time remaining, in the company of the SOS Brigade. As such, the group is taken on a whirlwind tour of summer activities, from swimming pools to summer festivals, part-time jobs, and bug hunting. Finally, Haruhi has made it through her entire list, and the last day of August arrives. Though Kyon tries to make up for lost time by doing his neglected summer homework, he eventually gives in to distraction, going to bed just before midnight.

Like the previous episode, Kyon's easy going summer vacation is interrupted as the group is taken on a whirlwind tour of summer activities, from swimming pools to summer festivals, part-time jobs, and stargazing. However, Kyon is wracked by a constant sense of déjà vu. Koizumi is likewise troubled, while Nagato seems even more detached than usual. They finally work out the truth: Haruhi has put the last two weeks of August into an infinite loop because she refuses to let summer end. They find out that the loop has repeated 15498 times. However, none of them can devise anything to stop the cycle from repeating. On the second-to-last day of summer, Kyon calls out after Haruhi in a last-ditch attempt to stop her from leaving unsatisfied, but fails to think of anything he can do, and she leaves. As the last day of vacation arrives, Kyon, resigned to the loop repeating itself, goes to bed without finishing his summer homework.

The same exact events in the previous episode once again unfold with slight variations and the gang wearing different clothes. Once again the Brigade, with the exception of Haruhi, finds out that they are trapped in an infinite loop. In this 15,499th loop, Kyon and the others once again are unable to discover how to escape the endless summer. In the same scene in the restaurant, Kyon is unable to stop Haruhi from leaving, hence dooming the world to another summer loop. Kyon again goes to sleep without finishing his homework on the last day of summer.

The previous time loop continues for the 15,513th time. This time, Kyon mentally senses an even greater familiarity with events and places from previous cycles. Once again, none of the Brigade members can devise anything to stop the cycle from repeating. On their final meeting in the restaurant, Kyon tries to call out after Haruhi but fails to do anything as visions of the loop's events flash in his mind. As Kyon regains his senses, Haruhi has left. As the last day of vacation arrives, Kyon, resigned to the loop repeating itself, goes to bed without finishing his summer homework.

The previous time loop continues for the 15,521st time. As the last day of vacation arrives, Kyon stays up late until midnight finishing his summer homework alone without any assistance. The final scene shows Kyon's clock ticking away on his desk but cuts out to the ending video one second before the clock strikes 12:00am.

The previous time loop continues for the 15,524th time. Kyon goes to bed without finishing his summer homework once again.

The loop continues for the 15,527th time. The scene where Kyon answers Haruhi's call loops twice, the scene in which Kyon notices Yuki at the pool loops twice, and the scene where Haruhi says she left the last day of summer open just in case also loops twice. Kyon once again goes to bed without finishing his summer homework.

The loop continues for the 15,532nd time. But as Haruhi leaves, Kyon knows what to do to break the endless loop of summer.