Asanagi no Aquanauts

One day, a girl asks Aki, the main character, to come out behind school building after school. He never expected such a thing would happen to him. Her name is Mio. She is a little bit strange, but pretty enough. "She'll tell me she loves me!" But he is wrong.... "Well, it's all your fault. Yes, there's no other way..." She pulls out a golden hammer. "You saw my secret!" Is this a dream? Or was what I saw the other day a dream...?