改編自日本真人真事的紀實文學小說。

罹患肌肉萎縮症的鹿野靖明，全身上下僅有頭部及手可以活動，只要出一張嘴，就可以解決所有生活大小事，其餘時間則是用這張得理不饒人的嘴巴，與母親鬥嘴。某天，鹿野對新來的志工美咲一見鍾情，於是拜託同為志工的醫學生田中幫忙自己代筆情書。但其實田中與美咲早已是男女朋友，三人就這樣展開了微妙的三角關係。隨著病情逐漸惡化，鹿野的生存能力漸漸被剝奪，然而，總是坦率耿直、對誰都同等對待的他，其實還有個無論如何都想實現的願望……。

Hisashi Tanaka is a medical student in Hokkaido Prefecture, Japan. He does volunteer work and gets to know Yasuaki Kano at the hospital. Yasuaki has suffered from muscular dystrophy since he was 12 years old. Yasuaki has his eccentric ways like running out of the hospital and saying that he wants to eat a banana late at night. Hisashi thinks Yasuaki is a lovely person. Yasuaki develops a crush on rookie volunteer Misaki Ando. He asks Hisashi to write a love letter for him, but Misaki is Hisashi's girlfriend.