The novel is an account of the mythologized legends around the Buddhist monk Xuanzang's journey to India during the Tang dynasty in order to obtain Buddhist religious texts called sutras.
The Bodhisattva Guanyin, on instruction from the Buddha, gives this task to the monk and his three protectors in the form of disciples — namely Sun Wukong, Zhu Bajie and Sha Wujing — together with a dragon prince who acts as the monk’s mount, a white horse. These four characters have agreed to help Xuanzang as an atonement for their past sins. Along the way they meet numberous obstacles that try to stop them from completing their spiritual mission. With their powers and commitment they will overcome all obstacles and reach enlightment.
Alot of the characters have at least 2 roles because of reincarnations.
I strongly suggest, to better understand the background of this story, to read some information about this literature. Read the chinese version in english not the abridged translated version, it's kind of askewed.