Patterned after Japanese art and silk screens, Taro, The Dragon Boy is an animated feature about Japanese mythology and cultures, focusing on Taro, a young boy who has to make a voyage to a distant lake to save his mother, who has been turned into a dragon.

In a world where dead people turn into books and are stored in the Bantorra Library where anyone who reads a book can learn their past, Bantorra Library is maintained by Armed Librarians who wield psychic powers and their enemy is a religious society known as Sindeki Kyoudan.

With another Wave happening in a week, Naofumi Iwatani and his party have no time to waste. However, when bat familiars raid Lurolona Village and the Wave countdown comes to a halt, the Four Cardinal Heroes reconvene with the queen, Mirelia Q Melromarc, for a quick briefing. The queen presumes that the odd occurrences are linked to the Spirit Tortoise—a threatening creature that has awakened from its slumber, back to cause havoc once again. A plan to put the Spirit Tortoise to rest is devised—but out of the four men, only the cursed Shield Hero agrees to help.

As the Holy Empire rises to power, the neighboring lands begin to gradually fall under its control. The Empire's conquest eventually reaches the small island of Erin, home to the Gael tribe. There, a priestess named Riannon is kidnapped to be offered as a living sacrifice to the demon king Arawn, a malevolent being rumored to have caused untold destruction in the past. Riannon's brother, First Warrior Arthur, rescues her, when Arawn suddenly materializes before them as a handsome grey-haired man. Hiding his true identity and remaining enigmatic, Arawn pledges his power and leadership to the tribe's cause and joins Arthur, Riannon, and their merry band of friends—including a talented swordsman, an agile hunter, and a group of ecstatic pixies—as they fight back against the Empire, while uncovering the dark secrets of the land along the way.