Koyomi decides to settle their argument once and for all, and this time, the winner of the argument would be determined through a game whose rules are to be dictated by none other than Koyomi himself.
As Karen steels herself for any obstacle Koyomi has prepared for her, Koyomi steps out of a room...
...and returns with Karen's toothbrush on his hand.
Karen shudders at the thought of having that shoved into her butt, but Koyomi assures her that he has other ideas. He reminds Karen that a person often allows only those whom they are familiar with to touch their hair, and he thinks that another activity that carries the same idea is brushing one's teeth, and a person can only tolerate so much of an unfamiliar person touching his/her hair or brushing his/her teeth before that person gives up. Thus, the game involves a test of Karen's psychological tolerance.
Five minutes of Koyomi brushing Karen's teeth is all it takes for Karen to get her chance to meet Kanbaru personally.
Initially, Karen finds Koyomi's "challenge" too easy, especially since Koyomi is no stranger to her, and is even disappointed about Koyomi's decision. Meanwhile, Koyomi simply prepares the start of his introduction to Suruga Kanbaru's fetishism.
As soon as Koyomi places the toothbrush into Karen's mouth, the simple activity of having one's teeth brushed becomes an orgasmic experience. Karen's body becomes jittery as her toothbrush explores every nook and cranny of her mouth, while Koyomi triumphantly grins at the sight of Karen's tolerance being broken down at each brush stroke.
One minute has passed, and Karen is close to breaking point, but Koyomi persists on. However, even he is slowly feeling the pressure as seeing Karen in an embarrassing situation is slowly having an effect on him.
Soon, the pressure becomes too much for Koyomi. As he continues to violate Karen's mouth with her toothbrush, she pushes back Karen, positions himself on top of her, and is just inches away from grabbing her breasts...
Then, Tsukihi appears. Fear overwhelms the two elder siblings.