This anime is a story about a girl who grew up during the war who works on writing letters 誰かの代わりに and experiences love and people's feelings.
I'm struggling to understand 誰かの代わりに in this sentence. A literal reading would be "instead of someone" but that makes no sense. I assume it means that she writes letters for other people, but I can't understand how it can mean that.
Also, the end of my translation -- and experiences love and people's feelings -- feels off. I'm not sure how to translate 知る here.