'Yui' which has already lost mother lives on three people of 'Maakun' of father and the younger brother. One day, as for the Maakun, Koto without Sei knowledge is made a fool of by a friend. When oneself does not tell the right Sei knowledge until a minute of mother, the Yui to see it is going to tell Maakun the Sex by oneself. However, thereafter the Maakun has got in the swing….
