Based on the manga by Inu.

By chance, Fukaya became the lover of a very quiet and beautiful girl, Fujino, yet he worries about the vibrator she's always carrying. "Perhaps, she considers me no more fondly than that vibrator?" In fact, their relationship began when he found her using a vibrator aboard a crowded train. Although they have passionate sex, he can't be certain that she loves him, and feels threatened by her vibrator. Now, the two of them are going to a pool. As she will be unable to use the vibrator in the water, he expects they will be alone, but...