From Yuu Translations:

Umi and Riku are members of their school's basketball club. One day, during a practice match, Riku gets his leg injured and isn't able to play in the match anymore. That's why he just sits and cheers up his twin brother... or that's what he was supposed to do, because the only thing he can think about is... his brother's butt?! He gets turned on and, after the match, asks Umi to do some naughty things with him in the school's warehouse!