Touma Inaba is a high school graduate who is trying to get into the prestigious Keio University. After dismissing a weird call girl the night before, he meets and falls in love with a red-headed woman named Mieko Yotsuba on the day of the entrance exam. When Mieko is listed for Keio, but Touma is not, he pretends to have been accepted, while having to enroll in cram school. In the meantime, he lives together with his cousin Urara Kasuga, who has a crush on him and is willing to do anything to help him (including being that call girl in the first chapter). Urara encounters Mieko and Touma at karaoke, but pretends to be his sister. However, romance rival Tatsuhiko Mashu suspects something is up, and after sleeping with Urara's friend Komi Natsuki, learns that Touma is not a university student. Mashu tricks Touma into revealing the truth to Mieko.
The heartbroken Touma starts a sexual relationship with Urara. The "happy couple" angers cram school teacher Kugenuma, who conspires to break the two up by framing Touma with harassment. The rumors force Touma to leave cram school, but Urara tries to work a deal with Kugenuma for him to return. But when Touma learns about the deal and that it involved sexual favors, he breaks up with Urara.
When Urara discovers that she is pregnant, she joins Natsuki at an escort service to raise extra money. Suspsecting that Mashu has cheated on her, Mieko makes advances on Touma, and they finally have sex. Urara, Natsuki and another girl named Mizuki hustle a client, but it backfires when the client then beats Urara up (and gang rapes Natsuki). Urara suffers a miscarriage, but reunites with Touma. When Mashu sees Mieko with Touma, he tells Touma to back out, but when he refuses, Mashu hatches a scheme where he has Natsuki give Touma a "memory-enhancing" incense as a study-aid. The incense is actually speed, to which Touma becomes addicted as he prepares for his next mock exam. Urara gives him an ultimatum to stop using the drug.
When Mizuki collapses and is hospitalized, Touma learns that she is his best friend Kouji's little sister and that she has been battling leukemia. When Kouji gets in trouble for shoplifting, Touma talks some sense into him. Urara takes on extra jobs to get a Christmas present for Touma, but when one of the hostess club patrons gets raunchy, she is saved by Ryuichi Kazama, a childhood friend who is ordered by Urara's father to be her home tutor and to secretly spy on them. When Ryuichi learns that Touma and Urara are living together, he kicks Touma out without telling Urara, and then tries to make a move on her. After Urara declines, Ryuichi conspires with Mashu to kidnap and to beat up Touma, but the plan goes awry when he pushes Touma in front of Mashu's car. Touma survives with a broken arm, but Urara does not know about it until later, as she breaks ties with Ryuichi. While Touma tries to recover, Akimoto learns Mashu is involved and attacks him, however, he is stabbed in the back and is hospitalized as well.
Touma's arm heals in time for the real exams; his test scores place him with S Shuu University, but he misses the school's deadline to enroll because of his attention to Akimoto's hospital bills. That leaves him with K Sawa and eventually Keioh, both of which reject him. Frustrated with his bad luck, he smashes the windows of his middle school, but the police arrest Urara, whose student ID was found at the scene of the crime. Urara agrees to take the blame and declines her college admission. Touma's "little sister" childhood friend Momoe Shimizu visits for spring break. Touma sleeps with the big-breasted cram school receptionist Etsuko Koiwai. He encountersKotomi Hayashibara, a twin-tailed high school student who idolizes Urara; she sleeps with Touma in order to frame him for infidelity. After the resulting fallout with Urara, Touma meets and sleeps with a shy, glasses-wearing Mieko Hotta. He eventually returns to Urara, who challenges him to ascend and descend the temple stairs a hundred times before agreeing to take him back. They must deal with her father's return.