Three years have passed after the decisive battle at the end of Yamato Takeru and peace has return to the land. Takeru, Oto and their friends return to the village and continue living their days in bliss until one night Shuranoo, piloted by Mikazuchi, marks a return and appears in the village to wreak havoc. Takeru is left without any choice and is forced to reactivate the sealed Susanoo from his slumber. However, this is just the beginning of a new threat, and Takeru has to overcome himself if he wants to rescue Oto from the impending danger.
The final two episodes were not broadcast but were released on video in 1997 under the title Yamato Takeru: After War. The beginning of a fourth season that never materialized, they are set three years after the original, with
Takeru now 16 years old, having to defend his homeworld from samurai-style robot invaders from planet Izumo in a "flying magic fortress." See also Little Prince and the Eight-Headed Dragon and the live-action version
of the legend released in the West as Orochi the Eight-Headed Dragon.
Source: The Anime Encyclopedia_ A Century of Japanese Animation.3rd Revised Edition