As soon as Jesus continues down the mountain with the disciples who had witnessed his transfiguration, the realities of the world are upon him. It starts with a healing of a boy. Others have tried to heal, to cast out the demon, but none were successful, and so Jesus does.
Then we witness a very human argument among the disciples, debating who is the greatest. Jesus then uses children as an example as to what true greatness is.
The chapter winds down with Jesus setting his face toward Jerusalem. Here he is preparing for the march to the cross and all of the trials that will come with this march. The disciples do not understand what he is talking about, and it is a struggle for them as well.