Our Mutual Friend: Charles Dickens

As the boat moved silently over the water, the thing at the end of the rope always followed. Sometimes it came near the boat and sometimes it pulled away like something alive. As it moved through the water, changes seemed to pass over the dead man's face. For the thing that followed at the end of the rope was the body of a drowned man.
At the inquest the dead man was identified as Mr. John Harmon, who had recently returned from abroad. John Harmon had not been drowned, but murdered. But how had he died and who had killed him?