Mysterious Girlfriend X

Ueno and Oka were caught kissing by Tsubaki, Ueno begs to him to keep quiet. Now all of Tsubaki’s thoughts are about kissing Urabe. Can he get that out his system, before he loses her to Ogata, who just asked her out?

Out of the blue, Oka has taken an interest in Urabe. When she asks Urabe to be her friend, Urabe tells her she doesn’t need friends. But that feeling might change once the two girls share a drink.

Tsuabki has finally worked up the courage to ask Urabe out on a date. But when he tries to bring it up, she speeds off. Will his dream of seeing her in a bikini come true or will it just stay a dream.

After having a conversation with Oka about first names, Tsubaki wants to call Urabe by hers. While waiting for Urabe, the girl from the picture he once carried in his wallet stops him to talk, and surprises him with a question.

After showing impressive display on the track, Mikoto is asked to participate in a relay race for the sports festival, although she rejects the idea of joining the track team as it means she wouldn't be able to walk home with Akira. As the students switch to their winter uniforms, Akira comes down with a cold. Having heard from Ayuko about how she dressed up in a bikini to help Kouhei get over a cold, Mikoto wears a bikini under her coat and gives Akira some of her drool, which helps him recover. On the day of the sports festival, Mikoto manages to help bring her team to victory in the relay race. Akira suggests to Mikoto that she should join the track team, but Mikoto once again rejects, as she much prefers spending time with Akira.

Akira has a dream where he got to squeeze Mikoto's breast and can't get the sensation out of his head. Taking a stroll, he decides to go in the direction Mikoto usually takes after school and comes up to an apartment block, which turns out to be where Mikoto lives. Mikoto invites Akira into her apartment for tea, where his imagination starts to get the better of him. After catching Mikoto as she trips after being spooked by some lightning, Akira almost reaches for her breast but Mikoto stops him. After getting Akira to tell her about his dream, she feeds him some drool whilst grabbing her own breast, giving him a stronger sense of the feeling. She then allows Akira the opportunity to touch them for himself, which causes Akira to become more aggressive and tease her ears until she notices Mikoto is crying and runs off. The next day, after Mikoto gets some advice from Ayuko about the new experiences she is feeling, Akira asks Mikoto to slap him as a way of apology. Afterwards, Mikoto explains the feeling she gets when her ear is touched before taking on Akira's bruise herself.

Noticing Mikoto with a serious case of bedhead, Ayuko fixes her up with a new hairstyle. Mikoto soon gets a lot of attention for her new hairstyle, which makes Akira a bit jealous. As they go home, Akira convinces Mikoto to go back to her old hairstyle and messes up her hair on her request. The next day, as Ayuko calls her out on it, Mikoto shows her the feeling of having her hair messed up, which she comes to enjoy. Meanwhile, as Akira gets frustrated when one of the male students starts selling photos of Mikoto's alternate hairstyle, he learns of an idol named Momoko Imai who has a strong likeness to Mikoto. Akira soon impulsively buys Momoko's photobook and enjoys looking at it. As Mikoto hears about it from Ayuko, who heard about it from Kouhei, she demands Akira to bring the book to her and destroys it, saying he doesn't need it when he's got the real deal.

Akira runs into Aika who, having just been dumped by her boyfriend, reveals how she knew of Akira's feelings for her when he lies about not having a girlfriend. Aika gives Akira the opportunity to try some of her drool, but he backs out realising it could damage his bond with Mikoto, bringing to light that he does have a girlfriend. The next day, Aika decides to approach Mikoto, inviting her to her school's culture festival on Sunday with the news that Akira might be there. Later that night, Aika calls out Akira with a fake wound on her face, manipulating Akira into coming to the festival with her and pretending to be her boyfriend. Akira tries to come up with an alibi in front of Mikoto, but she soon learns the truth from Ayuko. As Akira goes to the festival on Sunday, Aika shows up wearing her old middle school uniform.

As Ayuko goes to the festival with Kouhei, she spots Akira with Aika and decides to follow them. As Akira tries to hold off the advances of Aika, Mikoto appears in a box robot, suggesting they do a blind taste test of each of their drool to see which Akira prefers. Akira instantly gets a nosebleed from Mikoto's drool, due to the fact she was naked inside her costume. Aika gets naked and attempts to give Akira her drool, but she stops when she starts crying, making it clear that her advances on Akira was simply an attempt to get over losing her boyfriend, who had feelings for someone else even when he was dating her. Mikoto tries some of Aika's drool, sensing she has long had unrequited love but assuring her she'll one day find the right someone. Just then, Akira's blindfold gets shaken off and he sees the girls naked, prompting slaps from the both of them. As Akira and Mikoto leave together, she tells him that her eventual kiss with him will be her first, prompting a smile when she tries his drool.

As Akira is a little bashful around Mikoto, having seen her naked and all, she requests to try his drool, her experience how flustered he is. The next day, when Mikoto uses her scissors against Akira for attempting to hug her, she is shocked when she sees she had cut his forehead a little. As Mikoto takes Akira to her home to get his injury treated, she finds her scissors have become less accurate, which she believes to be caused by a strange dream she had after trying Akira's drool. As Mikoto gives Akira more of her drool, he starts picturing Mikoto naked and ends up banging his head whilst trying to avoid hugging Mikoto on impulse. After hearing his reasoning, Mikoto decides to hug Akira of her own accord, which in turn restores her scissor technique. The next day, as one of his classmates offers to replace Akira's gauze, Akira opts to keep it as it was the gauze Mikoto gave him. Later that day, Mikoto once again initiates a hug.

After enjoying a nice chat with Tsubaki’s sister, Yoko, Urabe learns more about her mysterious boyfriend family.