When Shinobu accidentally calls Yoko “onee-chan,” it starts the story of how the two met. As it turns out, it was much similar to accidentally calling her onee-chan. In elementary school, Shinbou was looking for her sister and Yoko takes Shinobu to Isami’s class.
The gags continue as Shinobu gets her math test back to find that she got a zero. When Aya accidentally says something wrong (it’s translated as fo’ sho’ in the subtitles), Shinobu thinks this will be on the test and starts appending it to the end of her sentences. To help Shinobu learn math, everyone decides to get together to study. But when Aya’s busy teaching Yoko, Shinobu goes to Isami. Isami asks for “one of [Shinobu’s] most prized possessions.” But Isami turns down Shinobu’s Venetian mask.