Leaving behind his grieving friends, Harry goes to Dumbledore’s office to place Snape’s memories into the Pensieve for viewing, witnessing Snape’s true life story. He sees Snape as a boy of nine or ten, observing Lily Evans, Harry’s future mother, first discovering her magical talents, and Snape clumsily trying to befriend her by telling her about her true nature as a witch. In a slightly later memory, Snape has become her friend and is giving Lily her first introduction to the wizarding world. Lily’s sister, Petunia, taunts Snape for his awful clothes, provoking him into attacking her, which angers Lily.
Harry observes a scene in which Snape watches Lily say good-bye to her family to board the Hogwarts Express for the first time. Lily reveals to Petunia that she knows that Petunia, who professes to hate magic, begged Dumbledore to admit her to the school. Petunia angrily accuses Lily of being a freak and a sneak, and Lily feels discomfort regarding Snape, who was the one who stole Petunia’s letter of response from Dumbledore. On the train, young James Potter and Sirius Black taunt and mock Snape.
Harry sees Snape’s disappointment at Lily’s being Sorted into Gryffindor rather than his own Slytherin. Later in their school career, Lily and Snape are still close friends, but Snape is jealous of James Potter’s attraction to her, and Lily disapproves of Snape’s Dark magic–practicing friends. Lily is furious with Snape for calling her a Mudblood, and for speaking contemptuously of Mudbloods in general. Snape frantically apologizes, but Lily tells him they have chosen separate ways.
Harry watches as Snape meets with Dumbledore on a wild hilltop. Snape has sworn loyalty to Voldemort but is meeting with Dumbledore secretly because he knows that Voldemort plans to kill Lily, her husband James, and her son, all because Snape told Voldemort about Professor Trelawney’s prediction that Voldemort would be killed by a boy born at the end of July. Snape promises to do anything Dumbledore asks if he will protect Lily.
Later, in Dumbledore’s office, Snape sobs for Lily Potter’s death. Dumbledore tells him that the son survived, and that Snape’s path is clear if he truly loved Lily. Snape agrees to devote his life to protecting Harry but makes Dumbledore promise never to tell anyone.
Much later, when Harry is a student at Hogwarts, Snape expresses his irritation at how Harry resembles his father in attitude and actions. Still later, Dumbledore and Snape discuss Karkaroff’s darkening Dark Mark, and Voldemort’s imminent return, which it presages.