Snape stepped in front of the gap between wall and frame, conveniently shielding its contents from view as he reached into the hidden compartment. Harry could hear the rustling of paper and the clinking of glass, before Snape withdrew a Pensieve, which Harry immediately recognized – though he kept that particular thought to himself. Snape placed the basin on the Headmaster's desk, and the portrait swung gently shut behind him.

“As I recall,” said Snape, “you are quite familiar with the use of a Pensieve.”

Harry could feel himself color slightly, but otherwise ignored the barb. He was, after all, rather too accustomed to insults from Death Eaters. With one hand deep in his pocket, his wand firmly in his grasp, he dipped his fingers into the bowl.

He could make out, in the corner of his eye, Snape's hand beside his, before he plunged head-first through the silvery liquid – and into the very same office they had just occupied. Here, however, a grave and weary Albus Dumbledore sat behind the desk – as it should be, Harry thought – with Snape in the same armchair where Harry had been, only it was drawn up within an arm's length of the Headmaster. Between them was a tray of unfinished tea, cold and presumably abandoned. Dumbledore swept it to one side with his good hand.

He very carefully rolled back the sleeve of his other arm. Harry gasped. The curse had spread; Dumbledore's entire lower arm was blackened and withered, the infection stopping just below his elbow. The Snape in the memory, too, looked taken aback.

“We did not predict it would move so quickly, yes,” said Dumbledore. “It is not a failing on your part, Severus; your skill is beyond reproach.”

“What happened?” asked Snape.

“I have been expending more magic than perhaps is wise. It is, however, necessary.”

“You are necessary,” said Snape fiercely. “It is parasitic; you should not feed it – you know that!”

“I am but one wizard, Severus, you –”

“You might as well tell me that the Dark Lord is but one Death Eater! Without him, there is no war, just as without you –”

“Without me, there is still Harry,” said Dumbledore, “and he is the child of prophecy.”

“He is exactly that – a child!”

“And he has held his own against Voldemort remarkably well in the past, all things considered. You should rather, I think, have a care towards the other child in your protection.”

“I have offered him assistance, which he has refused.”

“As you knew he would, Severus. He is frightened, and suspicious, and absolutely out of his depth – you, of all wizards, should not hold it against him.”

Memory-Snape looked away, as if stung. “Severus, I did not mean it as reproof,” said Dumbledore gently.

“Yes, I know,” said Snape, without feeling.

“Has he made any progress?”

“The Cabinet is still far from functional. The Dark Lord is not best pleased, but his current priorities are, fortunately for Draco, elsewhere.”

“Draco does remember the contents of your Vow, does he not?”

“Yes, of course. His mother reminds him of it often enough; she does not seem to understand that Draco is an adolescent boy, and therefore has a particularly fragile ego. He does not want to reveal weakness – which is not, I would remind you, a uniquely Slytherin characteristic.”

“No, it is not.” Dumbledore shifted his injured arm with a barely-concealed wince. “Do you believe, then, that in extremity, he has the capacity to commit murder?”

“It will not come to that.”

“No? Truly? You are usually significantly less optimistic.”

“It cannot come to that, Headmaster, because you cannot die. Not at this juncture, not with the Dark Lord still as powerful as he is.”

“And yet if he does not succeed, he – and his parents, undoubtedly – will be punished. As  someone who is intimately familiar with such measures, Severus, surely you would not willingly allow such a fate to befall your only godson.”

Out of the corner of his eye, Harry could see the real Snape's hands clasped so tightly behind his back that his knuckles had gone white. “I fail to see how I could prevent it,” memory-Snape ground out, “I can hardly countermand the Dark Lord's direct orders.”

“That is not what I ask of you, and I believe you do know this.” Something in Dumbledore's expression softened, and he added, “The choice between what is right and what is easy was never a simple one. You have chosen well, many times. I only ask that you continue to do so.”

“What happens, then, if neither choice is right?” Snape demanded bitterly.

“Think, then, of the Wizarding World. Think of the greater good – you are fast eclipsing me in value to the cause. I ask a great deal of you – I always have – but that is only because I am confident that you will make the right choice, in the end. Think upon it.”

“You ask for a murder, Headmaster.”

“You have done worse, Severus, in Tom's service,” said Dumbledore.

They flinched, both the Snape under Dumbledore's scrutiny and the one at Harry's side. “That,” whispered memory-Snape, his voice lashed with pain, “was uncalled for.”

“My apologies,” said Dumbledore, impassive. “I must, however, have your word. Will you do it, in his stead, if the need should arise?”

“If the need should arise,” said memory-Snape, looking for all the world as though he were fighting Veritaserum with every word, “if the need should arise, you have my word.”

“Thank you, Severus.” It was all that was left to say.

Memory-Snape rose from the chair, and did not bid the old man farewell before slamming out of the office. Fawkes alighted noiselessly on the desk, and laid his head on Dumbledore's burnt and withered hand. The phoenix began to cry, enough for both of them, but the hand remained as it was, dead.