Think of Deuteronomy as a sequel-slash-remake of the rest of the Hebrew Bible up until this point. It's got a similar cast of characters, a similar story, and even some of the same axioms. Moses is back, and once again we're talking about the history and future of the Israelite people.
This whole thing is made up of a bunch of flashbacks and flashforwards with Moses as narrator, and a few interludes from an outside writer. The book places the Israelites on the verge of entering the Promised Land while Moses stands before them to review all of God's laws. It's basically a pep talk—if pep talks involve a lot of finger-pointing and reprimanding. He wants them to have courage as they prepare to fight for the land they've been promised.
Once upon a time, the parents and grandparents of the Israelites now seated on the banks of the Jordan decided not to fight the inhabitants of the Promised Land. They were afraid of giants who guarded humungous castles in Israel and refused to go to war. When they finally decided to enter the ring of battle, God would not help them. Because of their lack of trust in God (and a number of other minor offences, traffic violations, and religious profanities), God refused to give them the Promised Land. Their enemies beat them down, and God made the survivors and their children wander in the wilderness for forty years. That generation died in the wilderness. Now their children are ready to conquer the land.
Moses sets up the showdown by reminding them of all the times God has helped them in their journey (1:6-4:40). He reviews a lot of laws with the Israelites, and illustrates what we call The Doctrine of the Two Ways. There are only two choices in life: good, which means following God's laws, or evil, which means not following God's laws. Goodness leads to reward. Evil leads to punishment. It's that simple, and Moses is here to make sure the Israelites get it. So what we get, basically, is an explanation of this covenant between the Israelites and God, with all the stories, specific legal doctrines, and ideas surrounding it. It's a big, mixed-up bowl of confusing Bibleness.
One of the main points of Deuteronomy is that once the people enter the Promised Land, they must not adopt the customs of the people they are displacing. In chapters 27-30, Moses encourages the people to be loyal to God and to avoid the idols of the Canaanites. "No idols" could almost be the motto of this whole book.
Finally, in chapters 31-34, Moses leaves the people with a farewell song and blesses them. The Lord doesn't let Moses enter the Promised Land, and the Israelites get a guy named Joshua to lead them in their battles. Moses dies, and that's that—the stage is now set for the next chapter of Israelite history.