Harry, Ron, and Hermione attempt to figure out who the Half-Blood Prince might be. Hermione suggests that it might be a woman.
In the evening, Harry goes to Dumbledore's office for his first private lesson.
The two travel by means of the Pensieve into the memory of Bob Ogden.
Bob Ogden was an officer working for the Ministry of Magic who has been sent to investigate an illegal instance in which magic had been used on a Muggle.
The perpetrator is a shifty, malicious man named Morfin Gaunt who lives in a small, dingy cottage in the woods near a manor house.
Once the Gaunts allow Bob Ogden inside, we find that they live in squalor.
The head of house, Marvolo Gaunt, is a mangy, mean man who abuses his daughter, Merope, and who refuses to cooperate with Bob Ogden.
Marvolo, who speaks in Parseltongue to his children, takes pride in being a pureblood wizard and the descendant of Salazar Slytherin. Merope wears a locket, a Slytherin family heirloom, and Marvolo wears a ring, another heirloom.
Bob Ogden is not impressed. It would seem that Morfin has put a hex on a Muggle.
The Muggle was Tom Riddle (sound familiar?), who lives in the big manor house near the Gaunts' rundown cottage.
When Morfin saw how his sister look longingly at the handsome Tom Riddle as he rode by on his horse one day, he was disgusted with her behavior and hexed the man.
The Gaunts are a pureblood family, and the thought of interacting with a Muggle is, to Morfin and Marvolo, sickening. They are full of prejudice and hatred.
Marvolo attacks and chases after Bob Ogden.
Harry and Dumbledore exit the memory, and Dumbledore tells Harry what happened next: Marvolo is arrested for trying to attack Bob Ogden and is taken to prison. Morfin is also sent to prison for using magic on a Muggle.
While they are gone, Merope uses her magic to make Tom Riddle fall in love with her.
The two run away, get married, and conceive a child.
While she is pregnant, Merope can no longer take knowing that her husband's love for her is the work of a spell. She releases him from the spell in the hopes that he will continue to love her without it.
He does not. Tom leaves Merope when she is still pregnant.
Merope's heart breaks.
Tom and Merope are Voldemort's parents.
Marvolo is Voldemort's grandfather.
With a sobered and saddened heart, Harry gets up to leave Dumbledore's office. On the way out, he notices the ring that Marvolo was wearing in Bob Ogden's memory. The ring is now cracked.