Stinky gets captured by strange men. Jan runs around and gets captured himself. While in prison, he makes a friend, Kor Kan, prince of a rival city. Actually, and this is a nice bit, Jan is pretty animalistic but Kor Kan befriends and cultivates him. It's a subtle step towards his civilizing process, because when you think about it, there are very few people that he's not prepared to kill on sight. Remember that up to this time, Jan has killed a few ape men, about thirty goblins, and an uncle. That's a pretty respectable body count. He was raised up to kill his dad eventually, via cannibalisation. Up to this time, the only thing he's been tolerant of has been Ramona, they're basically it. Apart from that, his only emotional bond is to a chimpanzee. Jan's stint in the prison of Set and his friendship with Kor Kan is a critical humanizing step.
Anyway, he fights a Mapinguari, escapes, and tries to rescue his bro. But he doesn't want to be rescued. This just gets him captured again. From there, it's a session in the royal gladiatorial games where, through cleverness, courage and freakish luck, he wins big time, and becomes something close to a Prince himself.