主人公长谷川天是一位广告作曲家，工作备受压力，精神生活无所寄托，村上雨音 (田中美里饰) 是普通银行职员，工作认真、性格安静，但每天不停地受上司、客人的气。某晚雨音在联谊上认识了吉田晴彦（及川光博饰）。吉田晴彦对雨音展开追求。雨音在电子信箱中发现了一封寄错地址的动人曲子，是由署名"HATA"寄来的。另一方面， 天发现自己送错了信，并在信箱中看到了署名"晴天娃娃"的信， 想起了消失已久的女友RINA。两人虽互不相识，却因此而展开通信于冷漠的电脑网络。但两人却估不到竟同于一座大厦中工作，更估不到的是两人从互联网络中互相倾诉而擦出火花。疑心极重的佳识，从其男友长谷川的冷漠感到会有不寻常的事发生。并想维系两人的感情，但是，佳识越来越难以理解的长谷。

Once in a blue moon

電子メールから流れ出た華麗な旋律。一通の間違いメールから二人は出会った。そうとは知らぬまま、二人の物語は進んでいく。メールの到着を告げるMSGランプに心が揺れる。そして、二人が本当の出会いを確かめあったとき......。

It all started when Takashi (Takenouchi), a talented composer who could not write a love song ever since his girlfriend Rina left him suddenly, accidentally sent an uncompleted piece of his latest work to Amane (Tanaka) via email. Deeply touched by his music, Amane wrote back to him, without getting a reply. Amane continued writing to him, pouring out her thoughts. Only when she signed as "Teru Teru Bozu", did Takashi responded with a mail saying "WHO ARE YOU?", because what Rina left him as a farewell gift was a teru teru bozu!