The book of Daniel begins with King Nebuchadnezzar, the ruler of Babylon, ransacking Jerusalem. He ends up taking back the cream of the crop, the most elite and noble Israelites, to serve at his court in Babylon. Daniel and his friends—Shadrach, Meshach, and Abednego—have to navigate between the demands Nebuchadnezzar is putting on them and their own religious principles. Ultimately, Daniel is able to stick to his guns—by avoiding un-Kosher Babylonian cuisine, for example—and proves to be of use to the king by helping interpret his dreams, like the one involving a particularly scary statue that symbolizes different world empires.
Daniel's friends are able to demonstrate their own faithfulness to God by surviving when Nebuchadnezzar throws them into a fiery furnace (nice going, Neb). Pop culture note: The saga of Shadrach, Meshach, and Abednego was immortalized (not that it wasn't already immortal) in the Veggie Tales episode "Rack, Shack, and Benny," in which Nebuchadnezzar (a cucumber—or maybe a pickle?) tries to make everyone worship a giant chocolate bunny. (Is your mind blown yet?)
After seeing God save Shadrach and Company, Nebuchadnezzar forgets the moral lesson he's just learned and keeps thinking that he's the center of the universe. God remedies this by making him go insane and wander on all fours in the wilderness with the animals for seven years. He recovers by finally acknowledging God as supreme.
Next, after Nebuchadnezzar ends his reign as someone piously testifying to the Jewish God, we see his grandson Belshazzar totally messing it all up again. At an out-of-control drinking and feasting party, Belshazzar and his wives and guests drink out of sacred vessels that were stolen from the Temple in Jerusalem. A hand—disconnected from any body—appears and writes a message on the wall in Hebrew. Daniel reads and interprets it for the guests and explains that it means the party's over: Belshazzar and his kingdom are both going to fall. And fall they do—Belshazzar gets killed that night and the Medes and Persians take over.
Next, during the reign of Darius the Mede, Darius' governors and officials scheme against Daniel and trick Darius into passing a law that condemns Daniel into being thrown into a Lion's Den for worshipping his God. Like with Shadrach and pals, God saves Dan, while the conspirators meet the grisly fate he had just avoided.
After the first six chapters, the trippy part begins. Daniel sees various visions, all of them symbolizing future events that are going to take place.