The main protagonist in Brideshead Revisited is Charles Ryder. When the novel opens up, Ryder is a Captain in the British army during World War II. Captain Ryder's regiment happens to come upon an abandoned mansion, which Ryder instantly recognizes as Brideshead, the ancestral home of the Flyte family, with whom he spent many years of his early adulthood. As Charles sees the home and wanders through the rooms, he recalls all of the significant things that he experienced with his university friend Sebastian Flyte, and later, Sebastian's sister Julia. Then, Waugh launches into a flashback to tell the story of Ryder's younger years in the present tense.
Charles and Sebastian first meet as undergrads at Oxford. Sebastian came from a wealthy family, and it showed. He had fancy clothes and cars, and never had to go without. Sebastian was also something of an eccentric. He always carried around a teddy bear named Aloysius, and frequently talked to the bear, brushed its hair, and did other things that would make casual onlookers believe that he (Sebastian) thought the bear was a living being. Charles was pretty much the polar opposite of Sebastian in that he was from a middle class family, didn't have any wealth to flaunt, and saw right through Sebastian's eccentric airs. The two quickly became friends.
The friendship between Sebastian and Charles was at first centered on alcohol. Sebastian liked to drink (eventually, he would develop a full-fledged problem with alcoholism), and Charles served as a mostly willing drinking buddy who was ready to go off with Sebastian whenever the fancy for a bottle of wine came upon him.
Charles and Sebastian's friendship soon deepens, and Charles is eventually invited to Brideshead to meet the Flyte family. Sebastian's mother is a devout Catholic who grates on her children's nerves with her constant harping on values and morality. Charles then realizes that most of Sebastian's actions are probably intended to upset his mother by going directly against her professed faith. Sebastian's sister Julia is quite a different story. She seems to be genuinely struggling with her faith, and can't decide whether she wants to embrace Catholicism or not. As a result, she has trouble making decisions in other areas of her life as well, which plays a big role later on in the story.
Soon, Charles gets entirely caught up with the Flyte family. He becomes the steadying influence that both Sebastian and Julia turn to whenever they need advice. He also becomes romantically entangled with both siblings at varying times, and this further complicates matters.
Eventually, however, Charles makes a break from the Flytes and their problems. Sebastian's drinking problem reaches a point where Charles can't stand to be around him anymore. Sebastian instead just spends time drinking and carousing with random people in random countries until Charles can't keep track of him anymore and essentially gives him up as a lost cause. Charles then loses touch with Julia when she decides to marry Rex Mottram instead of pursuing a relationship with Charles. Then, Charles himself marries a different woman, and gets on with his life.
That might have been the end of things except that Charles happens to run into Julia while they are both on an ocean liner crossing the Atlantic. The two soon start to reminisce and catch up, wondering if perhaps they made a mistake by not getting together when they had the chance. Whether they make up for lost time on the ocean liner or not is something I won't reveal here.
As I mentioned above, Brideshead Revisited ranks as one of my favorite novels of all time. The characters are richly drawn, well developed, and highly memorable. They all take actions that are consistent with the personalities Waugh has given them, and even though they exasperate the reader at times because of their stubbornness, everything nevertheless rings true. This is due in large part to the fact that Waugh does such a fabulous job with the dialogue and other nuances that help the reader get to know the characters on a pretty intimate level. At the same time, the characters aren't of the cookie-cutter variety, meaning that although their actions are consistent, they aren't always predictable.
I should also take a moment to say that Brideshead Revisited was made into a very successful miniseries that aired on both the BBC and PBS (I believe). Although the miniseries was critically acclaimed and is a fan favorite, I never quite warmed to it. I think Brideshead Revisited is one of those stories (like The Great Gatsby, another favorite of mine) that is much better on the page than it is on the screen.
Overall, I wholeheartedly recommend Brideshead Revisited to anyone looking for an interesting story with great characters. It's the kind of book that you don't want to end, but would rather keep reading about the characters for a long, long time.