The anime is based on a manga by Hiromu Arakawa (Fullmetal Alchemist, Silver Spoon), who is adapting the story from a novel series by Yoshiki Tanaka. Funimation describes the story:
In the kingdom of Pars lies the Royal Capital of Ecbatana, a powerful city ruled by the fearsome King Andragoras. Arslan, the 14-year-old prince of Pars, is a green ruler and lacks the fierce leadership skills possessed by his father. After his father falls victim to a treacherous plot headed by one of his most trusted comrades, the kingdom of Pars falls in the war with the neighboring nation of Lusitania. Narrowly escaping with his life, Arslan must embark on a journey to reclaim the fallen kingdom and fulfill his destiny as ruler of Pars.