Mika Tahara has just started high school and yearns to fall in love. However, to her dismay, a gal-like boy in her grade, Nozomu, acquires her cell phone number and begins to make frequent calls. When summer vacation starts, one day, Nozomu drunkenly phones her, but Hiro confiscates the phone and converses with her instead. Although Mika does not know who Hiro is, she feels at ease at the sound of his voice and the two befriend each other.

Mika and Hiro agree to meet each other when school starts, and to Mika's shock, Hiro turns out to be the delinquent boy she is afraid of. However, as she understands how gentle he is, they began to fall in love with each other and face a multitude of challenges threatening their relationship, such as Hiro's ex-girlfriend Saki hiring a group of men to rape Mika and continuing to harass her over the phone. An unplanned pregnancy occurs when Mika and Hiro consummate in the school library, and although they are determined to start a family together, a miscarriage brings tragedy to them both.

Shortly afterwards, Hiro discovers that his health is deteriorating and makes the painful decision to break up with Mika to keep his terminal illness from causing her pain, but continues to follow her whereabouts through his highschool friend, Nozumu. Mika eventually forgets Hiro with the help of Yu. Through a chance meeting, Hiro's condition is revealed to Mika and she makes a difficult decision to leave Yu to go to his bedside. Having Mika back, Hiro is determined to fight his disease and begins to improve. But during a routine checkup, Hiro's condition makes a turn for the worst and he exchanges goodbyes with Mika and his family for the last time.