Israel continues to conquer Canaan after the death of Joshua, but fails to completely drive out the Canaanites as God commanded. Those that remain behind turn Israel toward the worship of their idol gods (not those gods!), especially Baal and Ashteroth. That breaks Israel's covenant (not to mention hurts God's feelings), but God steps back and lets Israel learn from its mistakes, leaving them with no divine leader. Take that, Israel.
The last five chapters of Judges actually take place during one of these times, and they're not pretty. Israel becomes a barbaric nation ruled by whomever the biggest bully on the playground happens to be. There's rape, murder, genocide, and mass kidnappings aplenty in these chapters. Once God allows Israel to be conquered by oppressors, they realize how much they need him, so they turn from their idols and ask for his help again. He sends it in the form of warriors and champions called judges.
Each time God sends a judge, he or she leads Israel to battle against its enemies, and with heavenly help they restore peace and prosperity. But like clockwork, as soon as that judge dies, Israel turns to idolatry again. This happens with a few minor judges—Othniel, Ehud, and Shamgar—before the first heavy-hitter steps up to the plate.
Deborah, the only female judge, is a prophetess who, along with her general Barak (not that Barack), defeats a Canaanite king and his general, Sisera (with a little help from Jael and her handy-dandy hammer). She and Barak are so happy about this that they sing a victory song that lasts a whole chapter. Then they die, and the people fall to pieces again, worship other gods, get conquered, and come crawling back to God for help.
This time, he calls a man named Gideon to deliver Israel. Gideon rallies Israel around him and leads them to battle against Midian. But God decides that he wants everyone to know that it was he, and not some big army, that saved Israel, so he instructs Gideon to whittle away his army to just 300 men. With God's help, and a little band-geek ingenuity, Gideon's army defeats the Midianites and brings peace to Israel once again… until their next stumble.
Israel continues the vicious cycle through several more judges (most notably Jephthah, whose honor-bound sacrifice of his only daughter is one of the more depressing episodes in Judges, which is saying a lot) until we arrive at the main attraction: the story of Samson and Delilah. Passion! Deceit! Animal abuse! Samson's superhuman strength, fierce temper, and weakness for women build up to a fateful liaison with the duplicitous Delilah and a tragic, redemptive climax unlike anything else in the entire Bible—and maybe in all of literature. It's definitely the coolest of many cool stories in Judges, and is both the chronological and narrative climax of the book.
Note: As we mentioned, the last five chapters (17-21) occur chronologically sometime during the first chapter. We suggest you read them between chapters one and two so that the story of Samson and Delilah is the last thing you read. Judges is way more cinematic that way—and way less of a downer (more on that later).