After leaving Riderhood's house, the stranger heads into the London streets. At this point, the narrator of the book informs us that the stranger is none other than Mr. Rokesmith, a.k.a. Julius Handford.
As the guy walks around, he talks to himself, repeating everything that has happened to him in the past few years. We learn that he is none other than John Harmon, the heir to the Harmon fortune who's supposed to be dead! Dang, plot twist.
As we read on, we find out that John Harmon was drugged, robbed, and left for dead by Riderhood and his partner-in-crime, George Radfoot. But John Harmon survived and spent the next day or two in a daze. By the time he came to, he realized that everyone thought he was dead.
Wait a second. If John Harmon isn't dead, then whom did Gaffer Hexam fish out of the river at the beginning of the book?
It all started when people on Harmon's boat started to mistake him for the boat's third mate, George Radfoot. It turns out that Harmon and Radfoot looked a lot like one another. Harmon was nervous about marrying a woman he'd never met (Bella) once he was back in England, so he made a deal with Radfoot to get matching sets of clothes and to switch identities so that Radfoot could go check things out for Harmon before he (Harmon) came forward.
Once they reached land, Radfoot met up with Riderhood and gave Harmon some drugged coffee, hoping to rob him and leave him for dead. It's important to know here that Harmon was carrying a good bit of money on him when he left the ship, and Radfoot knew this.
After getting dumped in the river, Harmon washed up on some rocks and survived. Everyone he approached thought he was drunk, and it took him several days to recover from the drug.
We're not quite clear how George Radfoot died, but we can only assume that Riderhood killed him after they'd gotten rid of Harmon.
It was twelve days before John Harmon stumbled into the police station and saw the body of Radfoot, which had been identified as John Harmon's. At this point, Harmon decided that he didn't want the life his father had laid out for him. So he decided to stay "dead."
When he gets back to the Boffins' house, Harmon (aka Rokesmith) runs into Bella and takes the opportunity to tell her he's in love with her. Bella is insulted by this, since she thinks she's way too good for a secretary like Rokesmith.
She asks Rokesmith never to mention this love again, since it'll just make her feel awkward every time she visits the Boffins.
Harmon promises never to bother her again. And at this moment, once and for all, he decides to let go of the memory of his life as John Harmon.