The SOS Brigade is going on a summer retreat. And what better way to enjoy a summer retreat than on a deserted island? But if the mysterious island mansion doesn’t live up to Haruhi’s expectation, will she cause another incident? Or something worse?

Haruhi gets her wish for a murder mystery when the master of the house is found dead! But an actual murder may be more than she can handle. As they work to solve the case, things become more and more dangerous. Can they figure it out?

Hoping to reclaim the computer Haruhi extorted from the Computer Research Society earlier in the year, the Computer Research Society president challenges the SOS Brigade to a computer game they created for the cultural festival entitled The Day of Sagittarius III. Haruhi accepts his challenge, but soon finds out that playing the game is much harder than she thought. Yuki is initially mystified by the computer's operation but catches on quickly, as she will tend to do. Eventually, she manages to find out that Computer Research Society club members are cheating and is able to block and use it against them, thus winning the game and some laptops for the SOS Brigade club room.

On the day of the school's cultural festival, the members of the SOS Brigade are involved in different activities. Mikuru and her class open a fried noodles café, Yuki attempts fortune telling, and Itsuki is in a play. Kyon attempts to enjoy a normal day. Kyon goes to the auditorium, where bands are playing for the audience, and falls asleep. When he awakens, he sees Haruhi walking on stage in a bunny girl costume with a guitar hanging from her shoulders.