If this episode doesn't make you cry, you have no heart.
link
Konzen Douji was Goku's foster father.
For the happiness & safety of Goku, Konzen teamed up with two war generals & created a rebellion in heaven that ended in the deaths of thousands, all just to keep one child safe.
In the end, he sacrificed his own life to get Goku safely to the lower world. It was all in vain though, because his death left Goku permanently traumatized. The Merciful Goddess was kind enough to wipe his mind of all his memories so that he could spend his 500 year prison sentence in less anguish.
Konzen's last words to Goku were "I'll find you, & next time, I'll reach out for you." 500 years later, his reincarnation Genjou Sanzo, whom has no memories of his past life, frees Goku from his imprisonment.
Goku, while technically over 500, is biologically 18 by this point (16 in the anime) & has no father-son bond with Sanzo, so Sanzo ends up being his idol, savior, & the object of his infatuation. (It's not really clear is he's in love with Sanzo, or if he warships Sanzo as his own personal god).
Part of Sanzo's punishment for causing a rebellion in heaven is that he gets put into Goku's shoes & witnesses the murder of his own precious foster father; Komyou Sanzo.
