All Out!! follows the rough-and-tumble world of high school rugby through Kenji Gion, a short but fiercely determined freshman at Kanagawa High School. On the day of the school's opening ceremony, he meets the tall, timid Sumiaki Iwashimizu, and the two are invited to watch the rugby club in action. What Gion sees changes everything: rugby rewards more than height alone, and there are positions where players of every build can make a difference.
Convinced that he has finally found a sport he can fight for despite his size, Gion signs up on the spot, while Iwashimizu initially refuses to join. Gion will not accept his excuses and drags him back toward the club, setting the two on a difficult path of practice, teamwork, and self-discovery. For Gion, rugby becomes the place where his size stops defining what he can do, while Iwashimizu is forced to confront his own hesitation and decide whether he can stand on the field at all. As their training grows more demanding, the Kanagawa High School squad must learn to trust one another, endure punishing drills, and grow into a team capable of competing at the Hanazono national high school rugby championships, where the dream is to become champions.