“Little Sister Mei’s Devious Plan: It Can’t Be!”
“Imōto Mei no takurami, Misshitsu Daisakusen Sonna” (妹メイのたくらみ、密室大作戦 そんな)

Mei is Naru’s stepsister and hopes to take Naru back home. To accomplish this, she decides she has to hook Keitaro up with Mutsumi and it is in this episode that Naru finds out something shocking.