In Ch. 17 the Christians build a compound and a church and Nwoye finds himself drawn to what they are teaching, Okonkwo beats him but eventually gives up on turning the boy around.
In Ch. 18 the Christians admit the outcasts from the tribe, one of them kills the sacred python and later dies signifying to the villagers that the traditional gods are still alive.
In Ch. 19 Okonkwo returns to have a feast and re-enter society. Things seem good but there is a darker underside.
In Ch. 20 the white man's power begins to be shown as they settle a dispute (in their favor) and hang a member of the tribe that disobeyed their orders. This is sort of the beginning of the end.
In Ch. 21, Okonkwo realizes that things have changed forever and he cannot do much about it but he is seething inside.
In Chapters 22-25, Okonkwo is taken to prison along with the other elders and after they are released he hopes that they will fight back against the white men. But they will not and when he acts alone to cut off the head of the messenger from the court, he seals his own fate and that of the Igbo and ends up hanging himself in shame.