In this chapter, Abimelech conspires with the people of Shechem to kill the other sons of Gideon, but later destroy each other in their treachery.
Things have been getting gradually worse in Israel for some time, but this chapter is when things really start getting messed up. This chapter also really highlights the intertribal conflicts within Israel, as the men of Shechem support Abimelech because "he is our relative." In the last chapter we saw Ephraim complain because they hadn't been included in the fight against Midian, and in chapter 6 we saw how rapidly the men of Manasseh rallied to Gideon, who is their relative.
It's always been a bit of a juggling act keeping all the tribes aligned together. For another example, think of how the 2.5 tribes settled east of the Jordan created an uproar because they were seen as abandoning the other tribes (Num 32). Later in Joshua, the other tribes nearly went to war against these same 2.5 tribes because the transjordan tribes built an altar (Joshua 22). These are the kinds of problems that Moses and Joshua had to deal with, but now that Israel has no unifying leader, it now results in murder and open strife. Ironically, both Gideon and Shechem are part of the same tribe, Manasseh. So this isn't even intertribal conflict, it is between clans of the same tribe. We are going to see a lot more of this before Judges is over, and this is actually one of the reasons why Israel wants (and probably needs) a king, is to keep the tribes together.
After murdering all the other sons of Gideon (to prevent them from challenging Abimelech's kingship), Jotham pronounces a curse from Mount Gerizim, which was supposed to be the mountain of blessing (Deut 11:29). Through Shechem's treachery, it has become a curse instead.
For the parable that Jotham tells the Shechemites, it seems like the general idea is that "the trees", a metaphor for the people, were looking for a king, but all of the good plants (those that produce olives, grapes, etc) refused to rule over them, so they chose a bramble of thorns, which is a symbol of destruction and produces no good thing (see e.g. Gen 3:18). If there is any other particular allusion, I don't see it. What is obvious is that Jotham is predicting disaster, and that's what happens.
The short version is, if there is a people who deal treacherously with one man, they cannot be trusted with another. Since the Shechemites betrayed Gideon and killed his sons, it should surprise no one that they would also betray Abimelech in the right circumstances. I do think v. 28 is really weird, I don't understand why Gaal is referencing Hamor (who was the father of Shechem, and was killed by the sons of Jacob in Gen 34), since I would assume the inhabitants of Shechem are Manassites and not actually descended from Hamor. Maybe Gaal is implying that some (or many?) of the people of Shechem actually are descended from Hamor and living in servitude to the Israelites, and Gaal is encouraging them to rebel against Abimelech (who is an Israelite). It's certainly possible given that we know many parts of Israelites still contained native tribes, which would likely have merged at least partially with the Israelites.
The other thing that kinda confused me about this chapter is why, if Gaal controls the city and knows that Zebul is loyal to Abimelech, he would allow Zebul to remain. There must have been some reason why Gaal couldn't drive him out, or maybe Zebul was only loyal to Abimelech in secret, and publicly disclaimed friendship to Abimelech. Nevertheless, he appeared to remain loyal to his master.
The rest of the story is pretty straightforward. Abimelech beats the men of Shechem and destroys the city. Then he goes around the countryside and starts attacking other towns that probably allied themselves with Gaal such as Thebez. After burning the tower of Shechem he tries to do the same to the tower of Thebez, but dies, and the war is ended.
One other thing I'll point out is how Abimelech was afraid of being killed by a woman. Even though Deborah and Jael were considered heroic figures, this is obviously a very misogynistic culture. As in verse 49, the women of a town live or die depending on the fate of their husbands, who fight in their defense. But only rarely do they get to fight for their own survival.
Lastly, this chapter is pretty grim, but this is approximately what Israelite history looks like for the next ~900 years. It's sometimes good (like when Gideon overthrew the Midianites) and it's sometimes bad (like when the Shechemites killed all his sons). It's bad more often than it's good, but it's not the end of the story.