The Returning Favor of Haruhi Suzumiya is a very short (seven pages) text story found at the end of volume 12 of The Melancholy of Haruhi-chan Suzumiya manga. It is set in the normal Haruhi Suzumiya universe, but acts in part as a meta commentary on The Melancholy of Haruhi-chan Suzumiya.
Troubled at how the SOS Brigade's website still isn't drawing any e-mails or many hits, Haruhi Suzumiya comes up with the idea of running a manga which dramatizes the club's activities with fictional elements. She decides to use the same setting which Kyon suggested in The Sigh of Haruhi Suzumiya (i.e. Mikuru Asahina is a time traveler, Yuki Nagato is an alien, and Itsuki Koizumi is an esper) so as to both make the manga more exciting and prompt more e-mails of mysterious sightings. Kyon expresses concern that this will overlap too much with their movie, but Haruhi is confident that the manga creator's differing interpretation will make it stand out. Since no one in the SOS Brigade has the talent to draw manga (apart from Haruhi, who is uninterested in doing the work herself), Haruhi visits the Manga Research Society and requests the only member present in their clubroom to create the manga. They agree.
The first installment of the manga arrives in the SOS Brigade's e-mail a week later, and Kyon begins regularly uploading it to the SOS Brigade website. Haruhi and Koizumi both express admiration for the manga, and Nagato sets up a program to have the new chapters directly posted to the site.
Kyon becomes curious about the manga's author, so he questions a member of the Manga Research Society. He does not recognize the manga artist's penname and insists that on the day Haruhi recruited them, there never was a moment where only one person was present in the Manga Research Society's clubroom. Kyon recognizes that there are any number of possible solutions to this mystery, including that the Manga Research Society member he talked to was in fact the manga artist himself, that Haruhi wished the manga artist into existence with her powers, that she entered the wrong clubroom by mistake and recruited an artistically-inclined student who isn't a member of the Manga Research Society, and that Haruhi made the whole thing up and is secretly drawing the manga herself.
When the final installment of the manga arrives, the SOS Brigade honor the occasion by reading it together.