Years ago, Izumo saved a child named Ryuta from a youkai. After promising to return and adventure with him in the future, Izumo leaves Ryuta and forgets about his promise. By chance, Ryuta meets Izumo again. Over the years, Ryuta never forgot about Izumo and trained hard to prepare for their adventures. Izumo claims to have no memory of his promise or Ryuta, but that doesn't stop Ryuta from embarking on a journey with Izumo!
 (Source: Futekiya) 
Contains side story: Izumo no Shiawase (出雲の幸せ, Izumo's Happiness)