I've seen too many variations of the kung fu-skills-used-to-excel-in-non-combat-situations gag to find it funny anymore, but then, I'm old, and this cute manga is aimed at kids ages 9-12.
The similarly aged Kyuma is the last living member of his ninja clan, living atop a mountain and continuing to train and hone his ninja skills while awaiting the fulfillment of a prophecy that one day someone would come to him and need his service in battle. Meanwhile, a kids' baseball team gets trounced by older kids, and the team captain is told—by one of his players, who can read the future in a crystal baseball—that he will find the player he needs to win atop of Kyuma's mountain.
When the baseball player and the ninja meet, Kyuma naturally assumes that he's needed to fight for his new master, and that baseball is some form of ritualized combat between two opposing armies. From this initial misunderstanding, mild, grade school-aged hilarity ensues. Luckily for our protagonists, not only is the young ninja superhumanly loyal to his teammates, but most of his training—catching shuriken thrown at him, chopping firewood, etc—have equivalent actions within the sport.
Kyuma's training partner and best friend is his dog Inui, who wears a little ninja headband and also seems to have some pretty boss ninja skills (Inui throws the shuriken that Kyuma catches and dodges, for example). He's a pretty darling character design; a big, round pillow of a puppy, and the humans all have puppy-like proportions of their own, with big hands and feet they haven't quite grown into yet.
In this first volume, manga-ka Shunshin Maeda explores the initial premise, and then gradually introduces new characters, so that it isn't until the end of the volume that what seems to be the entire cast is assembled. I imagine younger kids will dig this book, but there was very little for me to grab on to. The volume was pleasant enough while it lasted, but I wasn't given much reason to look forward to a second volume.
But, again, I'm old.