A Sister's All You Need: As Long As It's For Research

Itsuki, Nayuta, and Miyako have a hard time dealing with the cold and head down to Okinawa. The next day, they visit a remote island so Itsuki can do some research and Nayuta decides to flail around naked in the water. Miyako is awed by what she's seeing and eventually jumps into the water naked, as well. Two days later, Itsuki is invited by his illustrator to go to Hokkaido, where they try various local cuisines. However, Itsuki finds out that Setsuna had been ignoring a deadline...