Our Mutual Friend opens with a grim scene: the discovery of a dead body in the river Thames. Documents indicate that the man is John Harmon, heir to a vast fortune amassed from dust heaps (mounds of discarded rubbish and coal dust, which could be both scavenged for valuables and resold to be recycled into new materials). His father, Old Mr. Harmon, was notoriously cold and greedy, and had alienated his son, leading to John Harmon living abroad for years. When he died, old Mr. Harmon's will had stipulated that his son could only inherit the fortune if he married a woman of his father's choosing. If John Harmon refused to do so, the fortune passed to Noddy Boffin, an employee who had faithfully served the Harmon family for decades.
With the ruling of John Harmon's death, Boffin and his wife are suddenly elevated to vast wealth and high social standing. The death also seriously impacts another family: the woman old Mr. Harmon had selected as a bride for his son is Bella Wilfer, the daughter of a modest clerk. Her prospects of becoming very wealthy upon her marriage are now destroyed, and Mr. and Mrs. Boffin show sympathy for her situation by having her come and live with them. The discovery of the body and the surprising transfer of the fortune also sends ripples of impact through a wide-ranging cast of characters of very different backgrounds. The body is discovered by a man named Gaffer Hexam who makes a living retrieving bodies from the river, but this particular discovery casts suspicions on whether he might have had something to do with the death, and those suspicions are publicized and promoted by another riverman, Rogue Riderhood. Two lawyers, Mortimer Lightwood and Eugene Wrayburn, are involved in the recovery of the body and the confirmation of the identity; as a result of this process, Wrayburn observes Gaffer's young daughter Lizzie, and is struck by her beauty. The rise of the Boffins creates a social stir among a circle of new money London families who cluster around the wealthy and socially ambitious Veneerings; it also impacts a street peddler named Silas Wegg, who is hired by Boffin to come and read to him. Last but not least, shortly after the discovery of the body, a secretive man named John Rokesmith first takes lodgings at the Wilfer house, and then is later hired by Mr. Boffin to act as his private secretary.
As time passes, the fallout from the Harmon murder seems to lead largely to greed, manipulation, and plots of betrayal. As a reward for his services, Boffin offers Silas Wegg the chance to live at the former Harmon residence, since he has now moved to a fashionable mansion, but Wegg uses this opportunity to search the property for something he can use against Boffin. Hexam is also found drowned in the river; his son Charley has been sent away to school, but has fallen under the influences of a schoolmaster named Bradley Headstone who becomes obsessed with Lizzie. Rokesmith has to use his position as secretary to try and protect Boffin from the many socially climbing individuals who would exploit him for his fortune; subplots demonstrate the greed and treachery of the fashionable world, such as when a couple named Mr. and Mrs. Lammle discover after their marriage that each has been trying to trick the other, and then go on to plot how to exploit vulnerable heiresses with the help of a con man named Fledgeby. Rokesmith proposes to Bella, but she refuses because she is preoccupied with marrying a wealthy man. Perhaps most alarmingly, Boffin is becoming increasingly paranoid and obsessive about his fortune, which leads him to cruelly mistreat his secretary.
Finally, Boffin fires his secretary for no reason, leading Bella to call him out for having become a cruel man, and leave the Boffin household. She and John marry, since she now understands that love is more important than money. After they have been married for some time and Bella has given birth to a daughter, John reveals two major secrets to her. The first is that he is actually John Harmon: on the night he returned to England he was drugged, attacked, and dumped in the river, but managed to survive. Previous to the attack he had switched clothes with another sailor, which led to the confusion of identities, and John took advantage of this opportunity to see what Bella was truly like. He and the Boffins also tricked her by having Mr. Boffin pretend to be greedy and cruel in order to see if Bella was truly a good person. Now that she has shown her faithfulness and integrity, the Boffins return the Harmon fortune to the young couple. Wegg's plot against Boffin is foiled, and he is cast out. Meanwhile, Headstone's jealousy leads him to attack and nearly kill Wrayburn, whom he has identified as his rival for Lizzie's affection. However, Lizzie is able to rescue Wrayburn, and the two marry while he recovers from his injuries. Furious that his plan has failed, Headstone attacks Riderhood, who was implicated in the near murder of Harmon and has been helping him with his plot to kill Wrayburn. Both men die while struggling together.
