Watch Saiyuuki Reload: Zeroin Online

Once, the land of Togenkyo was a peaceful land, where humans and yokai lived together. Then, however, men appeared who tried to revive Gyumaoh, the Great Yokai who was slain by the war god Nataku Taishi and buried in Houtou Castle 500 years ago, by using a forbidden combination of chemistry and magic The negative energy waves this created spread across Togenkyo, causing the yokai to go insane and attack humanity. To save the land, Kanzeon bosatsu orders Son Goku, Genjyo Sazno, Sha Gojyo, and Cho Hakkai to go west. The four set out on a journey to stop the experiment to revive Gyumaoh. On the way, they meet Hazel Grouse, who employs strange magic that uses yokai souls to revive the dead, and his follower Gato. The two of them join the party. And Ukoku Sanzo, a man they have a long history with, approaches in the shadows... Unforgettable characters, intense personal drama, and awesome action in a story about the bonds between four men!

Taking place in the past it offers a view at the Saiyuuki quartet before their formation and their journey to the west. First OVA starts with Sanzo and his master, also with Ukoku (known as a later Ni Jianyi). Second OVA shows first meeting of Goku and Sanzo. Third OVA considers the events after Hakkai's sins and his trial (that we know from first season of "Saiyuuki") and his decision to live in Gojyo's house.