Based on a movie under the same name.

From Nipponcinema:
General store manager Akio lives a carefree live in Okinawa with his dog, Kafu. One day he receives a strange letter from a woman named Sachi, but that’s not any person he’s familiar with. It turns out that the letter was a response to a joke wish he hung up at a Shinto shrine months earlier. Then one day, Sachi shows up and the two begin a relationship.