From MangaHelpers:

Ai is a fifteen-year-old girl who is living in the bad part of town, with her drunken, promiscuous mother. Her delinquent friends, Kana and Miki, regularly use Ai as bait to extort money from middle-aged men trying to buy sex. Ai feels horribly alone and trapped in a life she does not want.

Jun is a fifteen-year-old boy living in an elegant mansion. He has a teacher to give him lessons at home, and he studies the violin. He never leaves his home, though, due to his severe congenital heart disease. When any overexertion might lead to his death, playing it safe has always taken priority over interacting with other people.

Jun and Ai, both feeling confined and lonely, find each other through an internet social network on their cell phones. Soon, they have formed a deep friendship and possibly more. But will their affection for each other be able to overcome the extreme differences in their lives and upbringing? [tethysdust]