There was a time on a peaceful land where people and supernatural creatures can coexist in harmony. However, because of a calamity, these supernatural creatures began to run wild and wreak havoc. The reason behind the accident is actually caused by the revival process of a large mystical creature, King of Bull (Gyumaou).
The Heaven (Tenkai) which looked at the grieve situation summons a high rank monk (Genjou Sanzo) and assigns him to go to the west to stop the revival process of Gyumaou. The Heaven also assigns Son Goku, Sha Gojyo, and Cho Hakkai to join Genjou Sanzo to assist his mission.
The second installment of Saiyuki anime is also based on a well known story in China, Japan, Korea and Taiwan about a real journey of one Buddhist monk (AD 629) who travelled from China to India.
Descriptions taken from AniDB.net