How do I write a love letter?

Hiroko Watanabe has recently lost her fiancé Itsuki Fujii in a mountain climbing accident. On the day of his memorial ceremony, two years after his death, Watanabe finds his address in his high-school photo album. She decides to write him a letter. Surprisingly, she receives a reply from Fujii. Unsure who sent the reply, she cannot keep from writing again.