On the outskirts of a small village stands a massive tree whose canopy disappears far above the clouds. While grandiose on the outside, its interior houses the humble Mortem Post, which receives and delivers letters from those who have passed on. Deciding to break protocol to lend a helping wing, Rain, a flighty little courier takes a skittish girl by the name of Mila on an adventure through the inner workings of the post office. Together, they search for a letter containing the last words of someone who seemingly never existed.