Takuma has a heart disease, and probably won't live to be 20. But he doesn't know that. Ever since he was a child he would always be in the hospital. He wasn't allowed to do normal things, like playing basketball or going to school. When Mayu (Taneda Mayu) found out about his disease, she made him promise that he'll marry her when they would be 20. As the years past, Takuma and Mayu's love kept growing stronger.