Moses the man of God has passed away, and now his protege Joshua is taking over leadership and guiding Israel on their entry into the promised land. Joshua is rising to leadership on the cusp of something huge. There isn't much of a way to describe how big a deal it is that he's commanding the Israelites as they take possession of a entire country when previously they'd been slaves for 400 years, then wandered in the desert for another 40. It would be easy for Joshua to be overwhelmed by this responsibility, but God himself repeatedly tells Joshua to be strong and courageous, and offers him a major promise: for Joshua's entire life, no person or army will be able to overcome him or the nation he leads, and God will treat him just like he did Moses. So he's got that going for him.
The priests carry the Ark of the Covenant ahead of everybody and as soon as the Ark gets to the Jordan river, the water dries up and the whole troop crosses over dry land, in sort of a micro example of what Moses did with the Red Sea. Then, instead of attacking the fortress city of Jericho, they walk around it a certain amount of times for a certain amount of days, then blow their trumpets and shout and the wall falls down. But all's not right after this impressive victory because an Israelite man named Achan steals some accursed things and his whole family is stoned to death. That's the thing about big sins in the first part of the Bible--they usually result in death, and often result in extreme collateral damage in the form of innocents. Someone messes up and their kids always seem to pay for it.
There are a good many more battles and campaigns, most notably a battle against the Amorites when Joshua asks God to keep the sun from setting until they've defeated their enemies, and then it doesn't set for about a day. All told, the Israelites defeat thirty-one enemy kings in their battles. Then there are chapters and chapters of land division and inheritance allotment that are difficult to really follow unless you also have a map of ancient Israel to look at.
The book closes out on Joshua's death, where he speaks to the Israelites and delivers what is probably the most famous line in the book: "choose you this day whom ye will serve...but as for me and my house, we will serve the Lord" (24:15). He's been a strong leader for his whole life and, like Moses, the people have nothing wrong to accuse him of now that he's near the end. He basically tells them that he's not going to be around anymore and it's up to them how they behave, but regardless of what they do, he and his loved ones know who're they're going to follow.
Interesting things: 1. The tribes of Reuben, Gad, and half the tribe of Manasseh actually stay on the other side of the Jordan river because it's good cattle land and they have a lot of cows. The men go and fight for the rest of Israel, but then they go home to their families when the fighting is over. 2. Rahab may be the most amazing person mentioned in the book. She's a harlot (prostitute) in Jericho and she saves two Israelite scouts. She extracts a promise that her life and her family's lives will be spared in the oncoming attack, and they are. Then she marries an Israelite and ends up in the genealogy of King David himself and later, Jesus. This is one of those ultra-rare Old Testament examples of mercy and redemption. 3. Caleb, Joshua's fellow land-scout from forty years back makes sort of a guest appearance. He's as strong as he was as a young man and he has lived to take his inheritance in the new land.