The King of Youth - a spirit who seems to be an anthropomorphic personification of everything, both good and bad, to do with the concept of 'Youth' - captures Tripitaka. Not to eat him, as is already the default reason for spirits to do this, but so he can blackmail Heaven into giving him his father's kingdom. Heaven, he presumes, will not let Tripitaka die.
The king is brash, but he is also clever and knows powerful magic with which he defeats Monkey and Pigsy and prevents them from rescuing Tripitaka easily. His weakness is that he loves a mortal woman (though he also beats her in some scenes that I doubt they would get away with these days) and Monkey eventually uses her to get to him, kidnapping her and arranging a prisoner exchange.
The king tricks Monkey by releasing fakes of Tripitaka and Pigsy, but Monkey has a trick of his own and the King of Youth is captured by old Earth Spirits, who are jealous of his youth and power. They want to kill him, but Tripitaka forgives the king and this causes him to be saved.
My summary of the plot can't do this episode justice - the wit is all in the dialogue between the characters, and of course the narration.