Keitaro and Naru are rushing off to their exam class when they run into Haitani and Shirai. They have a part-time job working at a local amusement park and ask Keitaro how his relationship with Naru is progressing. They give him free invitations to the amusement park to help things out. Of course, they charge Keitaro 8,000 yen each for the tickets.
Back at Hinata House, Keitaro and Naru get the results from their last practice test and are shocked to find that Keitaro had made a B. To celebrate, Keitaro is going to offer the free tickets to Naru but the other residents interrupt and each think Keitaro is offering to go with them. Naru gets flustered and says she doesn't have time for such trivial things.
The girls chase Keitaro around the house, while Naru broods in the bath. Keitaro is chased into the bath with Naru ,and after the obligatory beating, he finally tells her that SHE is the one he wants to go with. The other residents have entered and ask Keitaro to explain himself. To appease everyone, Keitaro ends up buying them all tickets.
Naru is smiling and putting on makeup. It seems that since the amusement park trip ended up being a group event; Keitaro has asked her to the movies. Kitsune comes in and warns her not to give it up on the first date. Naru screams that it's not a date, and she has no intention of "giving it up".
Keitaro, meanwhile, has arrived at their destination three hours early and is waiting for Naru. She finally shows up, and she looks stunning. Keitaro overhears some people in the crowd wondering how he ended up with her.
They first decide to get something to eat. Keitaro points out the nearby beef bowl ramen place, but Naru refuses and they wind up at an expensive restaurant. They talk about their current grades and Naru plans to tease Keitaro by pretending to feed him. Unfortunately, Motoko and Su stop by at that exact moment. Su laughs and makes fun, and Motoko is disgusted. Naru, embarrassed, grabs Keitaro and they rush off to their next destination.
They go to a kung-fu movie, which causes Naru to wonder what made Keitaro think a girl would like that sort of movie. Naru, being naturally violent, ends up loving the movie, and inadvertently practices her karate on Keitaro.
Next stop on their date is the aquarium. They're surprised to see Tama-chan in a tank and they rescue her. They let some kids play with Tama-chan for a spell and are embarrassed when the turtle is told to "go back to your mom and dad". They come close to kissing, only to be interrupted by Seta and Sarah who had come to the aquarium to do a lecture on undersea archeology.
Naru starts daydreaming about what life would be like if they both attended Tokyo U. She starts to freak out and worries they're moving too fast. This is compounded by Keitaro grabbing her hand and leading her somewhere in a hurry. They run past a hotel and Naru remembers Kitsune's warning from earlier. She starts to panic, only to find out that Keitaro wanted to take a photo booth sticker together to commemorate one year since their first meeting. Naru realizes that maybe Keitaro isn't as perverted as she thought.
The Hinata girls are relaxing in the hot springs when they notice Tama-chan staring wistfully into the distance. Kitsune suggests Tama-chan senses a ghost, but Naru says that's ridiculous and that Tama-chan was looking towards the sea.
Back in the house, Naru invites Keitaro to study with her. This makes Kitsune suspicious about their hot date in the previous chapter. Meanwhile, Keitaro and Naru get distracted looking through her old photo albums. They find a picture of a very young Naru with a stuffed animal from about fifteen years ago. Naru still has the stuffed animal, Liddo-kun, and says she couldn't sleep without it when she was little. Keitaro remembers that was about the time he met his promise girl and he tells Naru that it would have been nice if she was that girl. Naru looks kind of sad and asks Keitaro if his promise girl was still the reason he was trying to get into Tokyo University. Before he can finish responding, Tama-chan interrupts. Tama-chan bows and then crawls away, leaving Naru perplexed.
Tama-chan then goes to Shinobu and Motoko and does the same thing. She tries with Su... but Su drools and asks her if she's given up and is ready to live in her tummy. Tama-chan crawls away rapidly. Tama-chan then encounters Kitsune and Haruka coming into the building and bows to them. Tama-chan then goes down the steps and leaves Hinata House.
Later, Naru tries to call Tama-chan for lunchtime but she's nowhere to be found. The residents all mention that she seemed different... almost like she was saying good-bye. Su finds a note the Tama-chan wrote, but unfortunately all it says is "Myuh". Naru reads the back of the instructions that Mutsumi had given them, and discovers that when hot springs turtles mature; they lose their ability to fly and will head back to the ocean. The gang decides to go out looking for her.
They search but have no success until Su unveils her latest invention: the Turtle Detection Radar! They manage to track Tama-chan all the way to the ocean, but then the signal disappears. They say their goodbyes, only to be surprised by Tama-chan flying around their heads. Naru looks at the instructions again and realizes that hot springs turtles don't mature until fifty years after birth. They all wonder what made Tama-chan come to the ocean and then she flies off again. They follow her as she lands on Mutsumi's shoulder. Keitaro and Naru are shocked to see Mutsumi again, but Mutsumi doesn't remember them.
After a second, Mutsumi remembers who Keitaro and Naru are and they introduce her to the other residents. The other girls head back to Hinata House while Keitaro, Naru, and Mutsumi go out for drinks. Mutsumi explains that she's come to Hinata Prefecture to take her entrance exam, and Keitaro proudly shows her the B that he's received. Mutsumi suggests that they become study buddies and the other two agree. After noticing their print club sticker, she asks if they've become a couple. Keitaro and Naru deny it, but Mutsumi asks Tama-chan and seems to understand turtlese.
Mutsumi gets up to leave, and Keitaro and Naru offer to walk her home. As they walk through the streets, Mutsumi reveals that she's not sure where she's staying. She has a map, but it's a map of Japan. After missing the last train out, Mutsumi convinces them to go bowling.
At the bowling alley, Keitaro and Naru try to study but are disturbed by Mutsumi who has never bowled before. Keitaro ends up helping Mutsumi; making Naru jealous. She complains that they ought to be studying, but that just makes the other two think she's a bad bowler. To prove them wrong, Naru pulls out her personalized bowling glove and bowls a strike. She goes to boast to Keitaro and Mutsumi only to discover they're now studying. Naru yells at them for not paying attention, and they all end up in a competition. Despite being a beginner, Mutsumi wins with a perfect score.
Daybreak arrives, and they realize that there was an address on the back of Mutsumi's map. As they reach their destination, they discover that it's right next door to Hinata House.
Naru and Keitaro leave their exam classes one day and run into Mutsumi on the street. She's a bit more pale than ususal, and explains that she receives a "Z" on her last practice test. She had even forgot to write her name. Not wanting Mutsumi to become a fourth year ronin, they form a study group and go to her house.
At Mutsumi's house, Keitaro and Naru marvel at how empty the place is and ask her when she's going to unpack. Mutsumi tells them she's already unpacked: there's a pile of watermelons on the floor. Keitaro opens a door and is almost run over by a train. Evidently, the train runs five feet from Mutsumi's house! As they prepare to study, Mutsumi brings out her secret weapon: a kotatsu(heated table).
Naru asks Keitaro a question about English, and Mutsumi asks them why they're so focused on English. Keitaro explains that the English section has the most points and is very important. Mutsumi appears sad and asks if it could be Turtlese. They tell her no. Later, they work on some multiple choice questions. Mutsumi performs her "Secret Study Method" which involves writing the numbers 1-6 on an edged pencil and then rolling it. Naru and Keitaro tell her that's fortune telling; not studying and that's possibly the reasons her grades are so bad. She almost decides to give up but the other two convince her to stay.
After a nap, the trio decide to grade themselves. To Keitaro and Naru's utter amazement, Mutsumi gets the highest score(485 out of 500). They ask her how she's managed to fail three years in a row. Mutsumi tells them that she's clumsy and she'll forget to write her name, or she'll faint before she can take the test. Keitaro and Naru are disheartened, but Mutsumi decides a change of scenery would be good. They end up at Tokyo U. and are inspired... until they find out that they've gone to the wrong university.
Mutsumi, Keitaro, and Naru are out for an early morning jog. As they finish, Naru plans to go home and wash off. They say goodbye to Mutsumi but she misunderstands and thinks they're going to go home and bathe together. They set her straight and invite her over to Hinata House and the hot springs.
When they arrive at Hinata House, Mutsumi notices that it seems familiar. When she finds out that Naru's room is on the third floor, she claims to know a shortcut. She moves a potted plant and unveils a hidden passageway. They ask her how she knew about the shortcut, but Mutsumi isn't sure.
Mutsumi and Naru prepare tea, while Keitaro daydreams about them serving him tea in the nude. They bring the tea in and find that Shinobu has also prepared some. They invite Shinobu to stay and study with them. Mutsumi tries to show Shinobu her Secret Study Method, but the others tell her she needs to teach Shinobu right. Mutsumi starts asking Naru and Shinobu personal questions about boyfriends and kisses. Thankfully, Kitsune interrupts and tells Naru that the bath is ready. As they leave for the bath, Naru grabs Liddo-kun and decides he needs washing.
Keitaro is left alone in the room and is shocked by Su and Sarah popping out of Naru's dresser. They've been exploring the secret passageway that Mutsumi found. They grab Keitaro and force him to go exploring with them. Keitaro has a sense of deja-vu as they explore the passages.
Unfortunately for Keitaro, the passageway leads into the hot springs where Mutsumi, Shinobu and Naru are bathing. Keitaro knows that Naru will kill him if she sees him, and Su and Sarah try to hide him. Meanwhile, Naru is bathing Liddo-kun and Mutsumi says she used to have one when she was younger. Mutsumi asks the two what kind of guys they like. Shinobu stammers about a guy who's nice and fun to be around. Mutsumi suggests someone like Keitaro and flusters Shinobu. Naru turns the tables and asks Mutsumi about her preferences. Mutsumi says she likes kotatsu and watermelons. Naru points out those are things; not guys. Mutsumi then says she would like a guy like Keitaro. Naru is indignant and tells Mutsumi that Keitaro "exceeds the baseness of the standard pervert". Shinobu says that he is getting better, and Naru agrees that it would be nice to go to Tokyo U. together.
Keitaro realizes that everyone is starting to like him, and that he'd better hurry up and get out. He makes it out of the bath, only to run into a naked Motoko. Motoko ties him up and reminds him of the taste of her blade.
As Mutsumi leaves to go home, Haruka seems to recognize her.
The gang at Hinata House is having a potato cooking party and they've invited Mutsumi to join in the fun. Mutsumi is serving the food, when she notices a big lump in the pot. It turns out to be Liddo-kun, whom Su had added because it looked so tasty. Naru, Keitaro, and Mutsumi get drunk and start singing the old Liddo-kun theme song. They each get a sense of deja-vu.
Mutsumi goes back to her place to get some more meat and vegetables. Keitaro follows her and Naru gets up to go also, but Keitaro tells her to stay and eat. Naru discovers they forgot their bags and goes after them. When she arrives, Mutsumi is on the floor and Keitaro looks like he is about to kiss her.(In reality, Mutsumi had fallen and Keitaro was helping her up.) She storms back to Hinata House.
Back at Hinata House, Naru is in a mood and trying to eat everything. Keitaro comes back and Naru snaps at him. She then goes to cut up some vegetables, while Kitsune explains to Keitaro that Naru had followed them to Mutsumi's house. He realizes what she must have seen and joins her in the kitchen.
In the kitchen, Keitaro tells Naru that he's not really trying to get into Tokyo U. because of his promise anymore. He's trying to get accepted to be with her. Naru yells and chases after him ,but according to Kitsune, this must mean she's feeling better.
After the party, Naru relaxes in the bath. Mutsumi joins her and tells Naru not to worry about her and Keitaro. Mutsumi had made a promise to someone when she was younger and this is why she's trying to get into Tokyo University. Later, it occurs to Naru how similar Mutsumi's story is to Keitaro's. She runs into Haruka who has now remembered why Mutsumi seems so familiar: There was a girl named Otohime who stayed at Hinata House years ago and she had a Liddo-kun like Naru's.
Naru gets a sinking feeling in her stomach. She runs to her room and checks Liddo-kuns's name tag. It's blank and she sighs in relief. Out of joy, she throws Liddo-kun into the air. When she catches it, she notices something scrawled on the bottom: Mutsumi Otohime
Naru checks her photo albums to see if she can find any clues. Sure enough, in the background of a photo, Keitaro-chan and Mutsumi-chan can be seen playing in a sandbox with Liddo-kun. Naru debates whether she should tell them. She also wonders why she would have been at Hinata House as a child.
After meeting Keitaro and Mutsumi for their study group, Naru decides that they're destined to be together and she shouldn't stand in their way. She puts on a ratty robe and glasses, and dresses Mutsumi in a low-cut gown. Hijinks ensue and Naru despairs of the two ever getting together. She plans to come out and tell Mutsumi.
She tries to tell Mutsumi, but Mutsumi misunderstands and tells Keitaro that Naru is in deeply in love with him. He confronts Naru and she ends up punching him.
Back at square one, Naru debates what to do.
Keitaro is dreaming about his childhood promise and is surprised when his promise girl grows into Mutsumi. When he awakes he shrugs it off to the fact they've been having so much fun studying together.
He goes Christmas shopping and runs into Haitani and Shirai. Mutsumi shows up while he's talking to them. They become furious with Keitaro for cheating on Naru, but he tries to tell them that he and Mutsumi are just study buddies. Mutsumi doesn't help matters when she says that she DOES like Keitaro because he's fun to be around.
Back at Hinata House, the residents are having their Christmas party. Keitaro arrives and Naru says she has something important to tell him. He tells her they'll talk later in the park because it's too rowdy right now.
After he's given everyone their gifts, Keitaro arrives at the park(accidentally bringing Liddo-kun with him). He sees a woman who he thinks is Naru and tries to give her the gift he had bought. The woman turns out to be Mutsumi. She notices Liddo-kun's Santa outfit and says he looks very cute. Seeing Mutsumi with Liddo-kun causes Keitaro to remember his dream from that morning. Mutsumi sneezes and Keitaro lets her borrow his coat. She looks at Keitaro, takes his glasses off, and kisses him. She tells him that her heart starts pounding whenever she sees his face. Naru appears next to them and says that of course it does...
Mutsumi is Keitaro's promise girl.
- Chapter 43: Invite Me, Keitaro!
- Chapter 44: The Happy, But Embarrassing, First Date ♥
- Chapter 45: The Sudden Goodbye
- Chapter 46: /looking Through 3000 Villages For A Place To Stay?!
- Chapter 47: Made Public! Mutsumi's Study Methods ♥
- Chapter 48: Talk Of True Feelings in the Nude ♥
- Chapter 49: The Disturbance, Liddo-Kun!! ♥
- Chapter 50: The Destined Two ♥
- Chapter 51: A Holy Night With That Girl ♥