From MangaHelpers:

Gengoro never held an interest in fishing even though his grandpa’s favorite hobby is fishing. He always thought it was a past time for retired old men. But when Minakami Haya, a pretty girl who’s the same age as him, moves in next door to his grandpa’s home from the country side, Gengoro begins to reconsider his initial judgment of the sport. Haya is an expert in fishing with an enormous amount of experience for her age. She's able to read the current, predict a fish’s movement and is knowledgeable about the mountains of equipment and techniques that come with fishing. But what Haya is after is the legendary Kaminotsukai or god’s messenger, a fish only heard about in stories that is said to appear every 100 years. [vyc]