Yumi-chan is an E-cup girl whose gentle appearance contrasts with her intensely erotic gaze. Wearing a gym uniform, she engages in cute yet sexy play using toys and rubber balls. Aichan, who struggles with swimming, receives special training from her at the pool. However, her flutter kicks underwater show little improvement, so they shift to a more erotic form of training. The underwater scenes grow increasingly sensual, deepening the bond between the two.