We are introduced to Yamada, considered to be her high school's idol, and to her mission in life in high school: to obtain a hundred sexual partners. However, she is rather picky, and decides to concentrate on a guy named Takashi Kosuda, whom she meets at a bookstore while shopping for a dictionary; and her would-be classmate. She then steps her campaign up a notch by finding a way to go home with Kosuda so that she can "get him." She is able to even get into his room; and seemingly with his liberated sister Kazuki's "blessing." However, her inexperience scares her to death that she rushes out of Kosuda's house in haste.

Yamada continues to seduce Kosuda by inviting him over to the pool. For this, she selects a rather sexy swimsuit for him. Yamada's friend Miharu Takeshita was worried Yamada might steal her boyfriend, Daisuke Matsuo, but it didn't happen--instead, Matsuo-senpai and Kosuda become instant friends. Yamada enjoys riding with Kosuda on the water slide, but gets disappointed when she finds out Kosuda is afraid of heights, hence him clinging tightly to her. Kosuda manages to make up with Yamada during the fireworks display, when he hugged Yamada from out of the blue, taking Matsuo-senpai's suggestion. Now Yamada prepares for "the big moment" by buying tons of condoms, while Kosuda seems even scared to buy condoms for himself.

When Yamada learns of Kosuda's membership in the photography club, and finds out that he is going to develop a few pictures in the dark room, she turns that into a sneaky scheme to "obtain his golden cherry." There, as Yamada accidentally trips the plug to the red dark room lights, Kosuda accidentally touches her bare breasts, but has no idea what he was touching. Disappointed, Yamada runs out of the room after Kosuda finds the switch to the normal room lights, but he catches up with her in his bicycle and offers her a ride home.

Jealous of her sister Chika bringing home lots of cute guys as boyfriends almost every day, Yamada steps up again to seduce Kosuda, just in time for Christmas Eve. Kosuda, on the other hand, gathered his guts again and asked Yamada out on a date on Christmas Eve, despite Yamada just waiting for him to ask her out. With Kosuda's gift of ear piercings, Yamada decoded this as a signal that they should look for a love hotel. Unable to find a cheap one, Yamada decides to go back down Love Hotel Lane to keep looking, until a nervous Kosuda attempts to kiss her, but his delays pissed a happy yet equally nervous Yamada off that she did the kissing herself, causing a massive headbutt. One way or another, Yamada's first kiss happened. Then her nervousness got the better of her that she ran away from Kosuda to hide somewhere.

As Mayu Miyano tries in vain to give Kosuda the perfect Valentine's day chocolate cake (she gives it to his sister instead, thinking it is in return for the apples she gave her), Yamada, while getting a mound of chocolates herself, struggles to make even one box of chocolate for Kosuda--and even her best were unpalatable to fish. Kosuda reveals to Yamada the next day that the chocolates given anonymously through the mailbox were "of a unique taste."

Looks like Yamada's obstacles in the way of Kosuda has multiplied by three--her sister Chika enrolls in the same school as her; and Mayu Miyano and Kyouka Kanejō, the girl who nearly beat her in last year's beauty contest, are now in the same class as she is! Setting Miyano aside for the moment, Kyouka is getting under Yamada's skin because of her charm and immense talent, envying her in the process, especially that she seems to be getting Kosuda's attention. Well, not that he is interested in her, but because she looks familiar, since she appears in the photography magazine he buys every month.

By now Yamada is battling a desperate "war" against the seemingly invincible Kyouka Kanejō. Kyouka even manages to beat Yamada, the fastest swimmer in her class, in a 50-meter freestyle swimming race, even with Yamada trying to cheat. Yamada even tries to enlist the help of her sister Chika, but when she took her to meet Kyouka, she and Chika became fast friends, much to Yamada's dismay. Meanwhile, summer break is coming, and Kosuda asks Yamada out to go somewhere for the summer break. Both see this as an opportunity, but there are hurdles.

It is three days of field trip to Kyoto for Class 2-H, and Yamada and Kosuda see this as another means of confessing to each other. But the task at hand, namely sightseeing for their class report; as well as the entire class being there, is what's keeping them apart--though they really want to get this thing over with. Even with Yamada and Kosuda having to hire their own boat (as they were left behind), their nervousness kept them from confessing to each other. In short, for them, the entire field trip was a waste and a disaster; and their only photo together was ruined by a rowdy, out-of-control Misato intruding (she was so rowdy she nearly ordered a beer, which was tantamount to expulsion).

As both Kosuda and Yamada are still reeling from what happened in Kyoto, a (not-so-) secret popularity contest among the boys (which results in a tie between Yamada and Kyouka) enables Kosuda to finally confess to Yamada in public. They were the talk of the town for a few days, and a real shocker for both Kyouka and Miyano. Yamada was pissed off with what Kosuda did, though she is secretly pleased with it. Only when both Yamada and Kosuda were appointed to the athletic meet committee were they able to talk and apologize to each other, nervousness and all, and despite Kosuda's denseness making Yamada angry. Yamada told Takeshita the news, but she only apologized in order to get laid, but Takeshita could see that she is in love with Kosuda. She is just running away because "she does not understand love at all, since she expected it to be fun." Kyouka, on the other hand, is boiling mad because, counting their School Festival beauty contest, Yamada beats her for the second time.

Both Kosuda and Yamada are going for another date, nagging worries and all. Kosuda is delayed unexpectedly, causing Yamada to go jumpy and grab the nearest guy she could get. Unfortunately, the second guy she grabbed was Keiichi. He then invited Yamada over to tea as an apology for what happened at the sports meet, with Kosuda, who caught up with them, looking like a chaperon. During tea Keiichi confesses to Yamada (and the more right now because he has heard from Kosuda that they are just dating), but gets rejected. However, her refusals get replaced by some deep thinking because, contrary to what she thinks of Keiichi, he is still a virgin (because Kyouka had all of those girls who have confessed to him terminated)--and that's what Yamada is looking for! Yamada, though, thinks Keiichi is just tricking her.

It's Christmas time again, and Kyouka invites everyone in the class to her family's grand Christmas party. Keiichi tells Kyouka to invite Kosuda over as well, much to her shock. Yamada has no interest in it at first, but gets lured in because of this "Secret Room" Misato is talking about. Kyouka has difficulty inviting her enemy Yamada, but it was solved when the latter demanded an invitation from her. The class, including their teachers, while awed by the party's grandness, finds it difficult to mingle with the rich people Kyouka also invited--save for Takeshita, who draws attention; and Misato, who lunges for Keiichi at will. Kyouka hears of this "Secret Room" thing from Misato, and she begins to suspect it could be her secret "Brother Room." Kyouka's panic arouses Keiichi's attention, so, as a result, she tells Yamada and Kosuda to get going, although letting her go seems to be a mistake.

Looking at her bed, with the box of condoms Yamada and Kosuda left behind still on it, Kyouka ponders whether they really did "it." Keiichi still looks forward to propose yet again to Yamada, hurting his sister's feelings. Meanwhile, with both of their siblings somehow able to help them about sex, both Yamada and Kosuda are able to say to each other that they want to "do it." They go to a love hotel on Christmas Eve, despite the usual obstacles. With a porn video as an aid and with Kosuda starting things up, things seem to go on smoothly this time, until Yamada accidentally turns on the rotating bed, which was malfunctioning, throwing Kosuda off the bed, fracturing his ankle in the process, and getting him hospitalized. Yamada then thinks of a way to help Kosuda.