Jeongguk Woo delivers mail as a postman in a post-apocalyptic world and dreams of riding the subway. When headquarters announces plans to revive it, Jeongguk is eager to join, but government worker Yonghan Cho personally bans him, citing the lack of dangerous missions in Jeongguk’s file. Infuriated by the snub, Jeongguk accepts the deadly task of delivering a letter to the AAA, a militarized area infamous for refusing mail. Once he arrives, he discovers the letter is addressed to Yonghan's sisters, who beg him to help them escape. Will he follow the postman rul