Tomo Aizawa and Junichirou "Jun" Kubota are the most inseparable brothers you've ever seen. They spar and roughhouse, but they have each other's back through thick and thin. But there's one minor problem: Tomo is madly in love with Jun! Jun hasn't noticed any of her approaches, even when she clearly confesses, because tomboyish Tomo has been simply another one of the boys since she was a kid. To make matters worse, he had no idea she was a girl for the longest time. Onnanoko, Tomo-chan! follows Tomo and her pals as they try to gain Jun's attention and escape the dreaded brozone.