This story is very similiar to "Mysterious Sign" as it involves the SOS-dan accepting an investigation request from a fellow student.
The story takes place in the lazy days of early March, the final exams are all over, and all that's left for the students is to wait until spring break (and after that, a new school year).
One day, a classmate named Sakanaka-san showed up at the club room and asked Haruhi for help with a certain problem: she thinks there might be a ghost in her neighbourhood.
Apprantely, Sakanaka is a dog-lover herself, and walks her dog (it's name is Rousseau, a West Highland White Terrier. Haruhi calls it "J.J.", for some reason) everyday. But one day, her dog suddenly began avoiding going into a certain area on their walk path. And after asking around, she finds that other dog-owners in the area are having the same problem too: their dogs won't go into that certain area either. On top of that, one owner's dog suddenly became mysteriously sick, and had the vets stumped.
Anyway, Haruhi took the usual preparations (dressing Mikuru up in Miko costume, and asking her to learn some exorcism spells) and set out with the whole gang to investigate.
Upon arriving at the scene, through J.J.'s help, Koizumi finds that the "dog repelling zone" is a circular area originating from the riverside. The gang heads there, and Haruhi got Mikuru to recite the
Heart Sutra (which, as pointed out by Kyon, is probably sacriligious, since the Heart Sutra is a Buddhism mantra, while Mikuru's Miko look is representative of the Shinto religion.
).
Soon after that, they encountered a jogger running past them with his dog. Surprised, Haruhi asked him how he got his dog to enter the area. The jogger said his dog didn't want to enter the area at first either, but after forcing the dog into the area a few times, the dog no longer fears the area anymore.
The "mystery" appeared to have been dispelled at this point, but a sudden twist came a few days later, when Sakanaka-san's dog also fell sick, with exactly the same symptoms as the other owner's dog.
The SOS-dan makes a second trip out to Sakanaka-san's place, there, Yuki finds out that the dog is infected with a type of alien mechanical lifeform (virus), which she called a "silicon-based coexisting type information lifeform element" (珪素構造生命体共生型情報生命素子). At this point, Yuki and Kyon began discussing the deeper aspects of how we define "life", as well as about the concept of spirits and souls.
(A memorable moment here is when Kyon asked Yuki whether or not spirits are actually real. Yuki, after showing a troubled look (that only Kyon can tell, of course), made an expression "like someone trying to hold back from laughing at a joke he's about to tell" (that only Kyon can tell, of course), and said
"That's a prohibited subject" ("それは、禁則事項")
(Yuki was copying Mikuru
))
After figuring out the problem, Kyon and Koizumi deliberated a plan with Yuki to "cure" the dogs by removing the virus from them (Yuki wasn't allowed to destroy them because, according to her creator, they are beneficial existences). To do that, Shamisen was required as a vessel to contain the removed viruses. The procedure was a success (which was performed under the guise of aromatherapy, to fool Haruhi and the dog owners), and the dogs became healthy again.
In the "epilogue", there was a discussion between Kyon and Koizumi about the implications of the incident this time. Kyon mentions the possibility that Haruhi's using her power again to cause odd events to occur again, but Koizumi denies that and says Haruhi's powers has not been used for a quite a while now. As a matter of fact, Haruhi has been
absolutely stable for the past few months, and that's
the problem. Since Haruhi's so stable, it's possible that some aggressive parties might be itching to cause some troubles (again) in order to get a reaction from Suzumiya Haruhi. The alien virus event this time may just be a sign or premonition to that.
This story really revealed more about Haruhi's personality than anything else. Also, we get a feeling that Kyon has really gotten to know Haruhi's personality after being together with her for a whole year.
Also, like the hints provided in the "epilogue", this story felt like a foreshadowing to another serious big event. Now, since Shoushitsu (vol.4) was a serious story about Yuki, and Inbou (vol.7) was a serious story about Mikuru, maybe we'll finally get a serious Haruhi-story in vol.9? ^^;