Saul gathered troops for battle against the Amalekites, whom they handily defeat.
Being a nice guy (sort of), Saul allows Agag, the king of the Amalekites, to live. He also keeps the animals alive because look how cute they are (1-9).
Because Saul disobeyed God, God tells Samuel that he regretted making Saul king. Angry about the situation, Samuel cries out to God about it all night (10-11).
As Samuel goes out to meet Saul, we see that he is setting up a monument. Saul seems to believe that he has carried out the command of the Lord. Samuel criticizes Saul for not killing the Amalekite king and destroying the animals.
Samuel and God believe that Saul has become too proud. When he was chosen to be king, he hid because he did not think that he was very important. Now he believes that he can break God's law and get away with it. Since Samuel cannot convince him that he has done wrong, he tells Saul that God has rejected him from being king. The sadness is about to get very real for Saul (12-26).
As Samuel turns to leave, Saul tears off a bit of his robe. Quick-thinking Samuel turns it into a symbolic lesson. He lets Saul know that the Lord has ripped the kingdom away from him and given it to someone else. And the worst part is, Saul doesn't even apologize for ripping Samuel's clothes (27-29). The jerk.
Saul asks Samuel to turn back and honor him before the people and elders of Israel. Samuel worships with him, perhaps to keep the kingship intact until a new king can be chosen (30-31).
Samuel asks for Agag, the Amalekite king, to be brought to him. Agag pleads with Samuel to let him live. Like a boss, Samuel kills him and splits his body in pieces (32-33). Samuel returns to Ramah and does not see Saul until the day of his death (32-35).