Late afternoon at school, Sumika and Ushio catch two girls kissing each other. When the next day's discussion turns to first kisses, Ushio gets the crazy idea of having a practice first kiss. Sumika, feeling a bit dejected that Ushio would rather practice on a model skeleton than her, and decides to take up karate again, having previously quit because she felt it was not cute. The next day, Ushio asks Sumika to meet her after school. Assuming Sumika had someone else in mind, Ushio lets her have her practice first kiss on her while wearing an Ultraman mask. As she does, they are watched by the two girls that Sumika and Ushio had seen kissing each other before.

On a rainy day, the gang discover a hidden note in a book referring to a certain thing, so they decide to search for it. After some investigation they find a key to a locker, but are reluctant to go to the boy's locker room, so Sumika gets Masaki to go for them. The locker contains another note mentioning a German person. With Azusa's help, they make the connection to the pictures of German musicians in the music room. This leads them to the creek (which in German is 'Bach'), eventually finding it to be a prank, but having fun all the while.