Harry actually has no idea who the Half-Blood Prince is. All he knows is he was the previous owner of an old textbook, which Harry inherited when he enrolled in Professor Slughorn's Potions class. Yates offers, "The book says it is the 'Property of the Half-Blood Prince,' but there is no name and no other record of him, so his identity is an enigma. But whoever he was, he was obviously very smart; he was capable of taking the conventional recipes for certain potions and spells and making them significantly better. He was an original thinker but also quite a dark thinker. The things he came up with eventually lead Harry into some very intense territory."

Handwritten along the margins of the Advanced Potion Making textbook, the notes from the Half-Blood Prince help make Harry even more of a star in Slughorn's class, which plays perfectly into Dumbledore's plans. He knew the Potions professor would try to "collect" Harry and bluntly tells Harry to let him. Michael Gambon explains, "Dumbledore knows Slughorn is hiding some important information about the young Tom Riddle, but he needs Harry to get him to reveal it."

Dumbledore believes that the key to Lord Voldemort's defeat lies in his past; therefore he has been gathering any memories he can of Tom Riddle, trying to glean when and how Riddle gained the knowledge that enabled him to become, as he says, "the most dangerous Dark Wizard of all time." Each memory he procures is carefully labeled and stored in a glass vial, including his own earliest recollections. Taking one out, he pours the contents into a floating Pensieve and shows Harry his first memory of Tom Riddle as a mere child.

Harry watches as a younger Dumbledore arrives at Wool's Orphanage. Production designer Stuart Craig says that the exterior of the orphanage was inspired by a building he came across while location scouting on the dockside in Liverpool. "There was this monolithic brick structure that dominated everything around it," he describes. "It was very sinister, very prison-like, and the design grew out of that. For the interior we used this glazed tile that was typical of Victorian institutions because it was durable and easy to clean. It has a dark, oppressive look that perfectly suited the environment we wanted to create for the orphanage."

Inside the orphanage, Dumbledore is led to a cheerless room and is greeted by the cold stare of the young Tom Riddle. The part of the 11-year-old Tom is played by Hero Fiennes Tiffin, who happens to be the nephew of Ralph Fiennes, the actor who portrays Lord Voldemort. "Tom is a dark, gloomy boy," says Tiffin, who was age 10 when he won the role of Tom Riddle as a boy. "He has these special powers and he can hurt anyone who is mean to him. He doesn't have any friends at the orphanage, so he steals other people's things because it makes him feel close to them. It's very sad."

"Hero was fantastic," Heyman states. "He is a lovely, charming boy, yet he was able to create an eerie detachment and a sense of control that I think is chilling onscreen."

"He's absolutely the sweetest kid you'd ever want to meet," Yates confirms. "He took direction so well. It wasn't difficult because he's quite charismatic, so it was just a matter of switching off any emotion and letting him be very still and very calm."

Dumbledore tells Tom that, at Hogwarts, he will be taught how to use and control magic. He leaves not knowing the wheels he has unwittingly set in motion, "but it is scary to realize that the die was cast all those years ago," says David Barron.

Dumbledore later shows Harry another memory, this one of a 16-year-old Tom Riddle, who has become one of Horace Slughorn's prized pupils. Frank Dillane, who plays the teenaged Riddle, notes, "Tom is very charming but very manipulative. His relationship with Slughorn is a bit topsy-turvy. I mean, in the student/teacher relationship, the teacher should be the one that commands authority. But from what we see, Tom is the one who appears to be pulling the strings."

"Frank did an amazing job of conveying that there is something brewing just beneath the surface," Barron says. "Tom is exceedingly polite, yet there is something vaguely threatening about him that obviously unnerves Slughorn."

On this particular evening, Tom lingers after one of Slughorn's gatherings and asks him about "a bit of rare magic." But Slughorn angrily cuts him off, refusing even to discuss such matters and ordering him out of the room.

Harry is understandably confused until Dumbledore explains that the memory is a lie. It has been altered by the one whose memory it is: Horace Slughorn. And whatever information Slughorn did impart to Tom could lead them to the only way to defeat Voldemort. Somehow, Harry must get Slughorn to overcome his guilt and his fear and divulge what, in truth, he remembers.

"This represents a real progression for Harry as a character," Yates observes. "Harry is fighting a war so when Dumbledore tells him that, with this memory, they could defeat Voldemort, that's all he needs to know. Killing Voldemort is what primarily drives Harry, so Slughorn becomes just a means to an end. It is a definite departure to see Harry Potter working this guy to get what he wants."