Aktha and Supes's dragons go on a deadly rampage under Pytonpyt's control. Thaim and Aktha must travel to Ancient KOTOWARI and uncover its hidden truth in order to stop Pytonpyt's invasion.

Thaim is devastated by Nagi's sacrifice. His dragon's roar suddenly pierces his ears, awakening to a new power. Meanwhile, the evil force in Upananta begins its destructive transformation.

The touching moment of Nagi's reunion with her mother is cut short by a vicious intruder. With the attacker at their heels, Thaim and Nagi head to the island where Aktha is hiding.

Secrets do not stay hidden forever. Saila's last-ditch effort to save the world reveals the long hidden truth of Upananta. However, some secrets should never be revealed. Unfortunately for Thaim, he has just uncovered a deadly one.

Thaim finally finds Aktha only to discover the great hero is not the same man he once admired. Aktha even turns down Thaim's request to return and save Upananta. Unable to accept Aktha's refusal, Thaim challenges the powerful hero to a duel.

Nagi is astonished to see Thaim suddenly appear out of nowhere. Although she was wary at first, after learning why he came to her world, she joins his quest for Aktha. However, an unpleasant surprise awaits them.

Thaim believes Aktha is still alive. He and the eccentric Saila begin to unravel the hidden truth of another world in their search for Aktha. Back in Yokosuka, Nagi also seeks to uncover the secret behind her dreams. When she steps into her mother's untouched study, she has a mystifying encounter.

Nagi is a pessimistic teenager troubled by dreams of Upananta, a fantastical world where fish swim in the sky. This world is on the brink of destruction, and it has just lost its greatest dragon-riding hero.