One night, a crankypants riverman and his daughter find a body in a river outside London. Dum dum dum. The body is that of John Harmon, a young man who was supposed to return to England to claim a huge inheritance from his father. But now that the young man's been murdered, the money will go to a pair of old, naïve servants named Mr. and Mrs. Boffin. Right on cue, whole bunch of shifty folks come out of the woodwork to see what they can get out of the Boffins.
One of the points of Old Man's Harmon's will was for his son, John, to marry a young woman named Bella Wilfer. Well it looks like Bella Wilfer won't get the money she expected because her would-be husband is dead. But the Boffins still want to take her in and treat her like their adopted daughter. Meanwhile, two guys named Bradley Headstone and Eugene Wrayburn compete for the love of Lizzie Hexam, the same girl who found John Harmon's body in the river at the beginning of the book. As the plot unfolds, Bradley's hatred of Eugene gets deeper and deeper until he attempts to murder the dude. But the murder is unsuccessful and it only brings Eugene and Lizzie closer together as Lizzie nurses Eugene back to health.
About halfway through the book, we learn that John Harmon isn't dead after all. Instead, he's posing as Mr. Boffin's secretary, Mr. Rokesmith, so he can judge whether Bella Wilfer would make a worthy wife or if she's interested only in money. Eventually, Bella proves herself by sacrificing her position with the Boffins in order to defend Mr. Rokesmith. Harmon thinks this is proof enough of her value, so he marries her and eventually gets around to reclaiming his former identity (and the fortune that goes with it).
By the end of the book, all the bad guys are punished (yay!) and the good guys are rewarded (double yay!). A conniving moneylender named Mr. Fledgeby gets beaten up for being a jerk and the would-be murderer Bradley Headstone drowns in a river with another jerk named Mr. Riderhood. John Harmon and Bella Wilfer move into a fancy house in London and take the Boffins as their trusted servants. Eugene Wrayburn marries Lizzie Hexam and everyone seems pretty happy. Huzzah.