Harry remains kneeling by Snape's body, when Voldemort's cold, projected voice informs them that if they continue to resist, everyone will die. If Harry doesn't come out to meet Voldemort in the Forbidden Forest within the hour,Voldemort will come and find him, then kill everyone else.
Ron and Hermione don't want Harry to go, and they hurry him back to the castle.
Harry's still in shock following Snape's death.
The castle is silent; everyone is in the Great Hall, where the injured and the dead are being tended to. The Weasleys are huddled around Fred's body.
Next to him are the corpses of Lupin and Tonks.
Harry is overcome. He can't believe all these people died for him.
He runs up the stairs to the headmaster's office, and pours Snape's memories into Dumbledore's Pensieve. Harry allows himself to slip into the pool of memory.
…And suddenly we're in a playground, on a sunny day. We're inside Snape's memories – who knows what we'll find out here?
We get a glimpse of a young, scrawny, neglected-looking boy – nine-year-old Snape – watching two other kids play on a swing set. It's Lily Evans and her sister, Petunia (yes, Aunt Petunia, of Privet Drive fame). Lily is magical, but Petunia – you guessed it – is not.
Snape emerges from his hiding place and tells Lily the truth – she's a witch, and he's a wizard.
Snape and Lily become friends, as he explains Hogwarts to her. He's clearly infatuated, and tells her that it doesn't matter at all that she comes from a Muggle family.
We witness some tense scenes; Petunia resents Lily for being gifted, and her reaction is to call her sister a freak.
On the train to Hogwarts, we see some other youthful, familiar faces – James Potter and Sirius Black.
Snape tells Lily that he hopes she'll be in Slytherin, and James and Sirius deride him. At Sorting, Lily is Sorted into Gryffindor immediately, and Snape to Slytherin.
A few years later, Lily and Snape seem to be arguing; they're still best friends, but she can't understand his choice of other friends – they seem to be the young Death Eaters club of the time.
He turns on some of the members of her House – James, Sirius, and Lupin. He's already figured out that Lupin's a werewolf, and he's suspicious of all of the boys.
He's mostly concerned about the fact that James likes Lily, but she reassures him that she thinks he's an arrogant jerk. Snape calms down, and the fight is over.
Next, we witness a scene we've seen before, in Snape's memory in Book 5, Harry Potter and the Order of the Phoenix. James torments Snape, and Lily steps in to defend him – but Severus, totally humiliated, calls her a Mudblood, an unforgivable insult.
Later, Snape tries to apologize, but Lily won't hear it. She's done with him, and with his Death Eater friends. Their paths have diverged, and there's no going back.
The next memory takes place years later; adult Snape is on a desolate hilltop, talking to Dumbledore. Snape tells Dumbledore that he told Voldemort about Professor Trelawney's prophecy; Voldemort then decided that it meant Lily Evans (now Potter) and her child – Harry.
Snape is desperate to prevent the death of the woman he (still) loves.
Dumbledore is disgusted by him for telling Voldemort in the first place, but agrees to keep the Potters safe. Snape agrees to do anything in exchange for this service.
Later, Snape and Dumbledore are in Dumbledore's office. Snape is distraught by Lily's death, and Dumbledore tells him that her son survived – and that his eyes are just like hers.
Snape agrees to help Dumbledore protect Harry, even though he's James Potter's son too, on the condition that nobody ever find out.
Later, in Harry's first year, Snape complains about Harry's arrogance – he thinks the boy is just like his detested father.
At the Yule Ball during Harry's fourth year, Snape informs Dumbledore about the gathering of the Death Eaters. Dumbledore praises Snape's courage… maybe he shouldn't have been a Slytherin after all.
Next, we see Snape tending to Dumbledore, who's tormented by his blackened, destroyed hand. Snape gives him a reviving potion, and Dumbledore regains consciousness. Snape does his best, but the most he can do is gain Dumbledore another year – the curse of the Horcrux ring (which inflicted this injury on Dumbledore) is too strong for a full-on cure.
Dumbledore makes a plan for Snape. Voldemort has decided that Draco Malfoy will kill Dumbledore, but more likely, will die trying.
Dumbledore asks Snape to step up and take Draco's place, to save the boy from Voldemort's wrath – Dumbledore wants Snape to kill him himself. Unwillingly, Snape agrees.
Finally, near the end, Dumbledore explains what will happen to Snape: when a time comes when Voldemort won't let his snake, Nagini, out of his sight, then Snape can tell Harry the truth.
The truth? Uh oh.
The truth: on the night that Lily died for Harry, a part of Voldemort's own soul was severed when his Killing Curse rebounded on him. That fragment of Voldemort's soul lives in Harry, which is why the boy has his strange connection with the Dark Lord.