PART ONE: THE MEMORY
Snape entered the loo, only to be assaulted by the sight of Gilderoy Lockhart emerging from a stall. Snape glanced at the toilet he'd just used, and realized with disdain that it was struggling to flush.
"You stopped up the toilet," he noted dryly.
Lockhart looked flustered. "Actually," and before Snape could react, he'd whipped out his wand, "Obliviate!"
Lulled into the spell, Snape could only listen and accept it as Lockhart informed him, "You stopped up the toilet."
After the other man's departure, the trance crept from Snape's mind, and he glanced, befuddled, at the toilet he'd stopped up, wondering why he still felt like he needed to use the bathroom.
Ah, I remember that, Snape thought, irritated by that half hour he'd spent searching for Filch to unclog that toilet.
He was still under the lull of the memory restorative draught. When testing their concoctions, the NEWT Potions class had yielded a surprising number of students who had been obliviated at one point or another by Gilderoy Lockhart. His curiosity piqued, Snape had swallowed the draught himself as soon as the class had filed out. He'd already witnessed Lockhart knocking down a shelf of his potions (he'd known he wasn't that careless!), slipping on the slick surface outside the DADA classroom, and swallowing his coffee the wrong way. Why that infernal idiot had felt the need to erase the recollections of even his most minor embarrassments still baffled him. After all, the man had humiliated himself on a daily basis with his pitiful attempts at wizardry.
Snape would have hexed the imbecile if he wasn't already sitting in Saint Mungo's, struggling to recall the slightest detail of his existence. As things were, he was beginning appreciate the concept of cosmic justice. How fitting that a man with so little respect for the recollections of others had been robbed of his own recollections.
He could feel the power of the draught waning, so he prepared to rise; he supposed there were no more Lockhart incidents obliviated from his mind...
And he collapsed back to his seat as something much more powerful broke through, another memory, buried much deeper, obliviated by a far more powerful spell.
The draught wasn't usually strong enough to restore very powerful obliviates (hence Lockhart's continued presence in Saint Mungo's); Snape recognized the fleeting quality of the memory as it tugged at his thoughts. It had been buried very deeply. Clearly someone really wanted him to forget this one.
Intrigued, he cleared his mind, rallying every Occlumency skill he'd ever acquired. He forced the questions out of his mind-- Who had done this? Was it Voldemort trying to hide some nefarious scheme? Potter trying to hide some miserable little escapade?-- no, Potter had neither the skill nor the guile to pull it off-- and at long last his mind grew quiet.
The memory trickled back slowly at first-- Potter thrashing on the bed, Dumbledore's sad blue eyes, Snape's own white hands stirring a potion-- then it overtook him with an explosion of color and sound:
"How long has he been in this state?" he heard himself ask. He was standing over a steaming cauldron in the Hospital Wing, in the final stages of brewing a potion.
"Three hours," Poppy Pomfrey replied, hovering worriedly at the foot of Potter's hospital bed as the boy thrashed and quivered in his sleep. "He won't even respond to stimuli now."
Snape sniffed the potion, verifying it was in the proper state for ingestion.
"Severus," he could hear Dumbledore's gentle voice, feel his hand on his shoulder. "Is it ready?"
"I've prepared the counter-poison as per your request, Headmaster," he replied, watching the boy on the bed struggle against an unseen opponent in his fever-ridden state. "It awaits only the blood of a relative. You have a sample on hand, I trust?"
"No, Severus. They would not consent."
Snape glanced back sharply at the aged headmaster looming behind him.
"Without the blood of an immediate relative," he explained carefully, wondering if the situation had addled the Headmaster's wits, "this potion is utterly useless. Potter's as good as dead. Did you explain this to them?"
Dumbledore looked sadly over at Potter, still unconscious on the hospital bed, then back to his Potions Master.
"I want you to use your own blood, Severus."
Snape stared at him. "Have you heard a word I said? This potion will not--"
"Use your own blood, Severus," Dumbledore repeated softly.
Snape followed Dumbledore's orders, wondering what in Merlin's name was wrong with the other man. This would do nothing to help his wonder boy; the Headmaster was idling away the scant time before the boy inevitably succumbed to the poison. Dumbledore's logic defied Snape.
He was unaccustomed to disobeying Dumbledore's orders, however - even if the Headmaster was operating in a state of intense denial - so he made a careful incision and watched his blood trickle down his pale arm and dribble into the potion. He stood back as the older wizard carefully ladled out a vial, then poured a potion as useless as a placebo down the dying boy's throat...
And suddenly Potter's body stilled, the sharp, rasping breath fading into a calm, steady rhythm. A look of peace slackened over his features, the color rushing back into his pale face. Dumbledore pressed the empty vial back into Snape's numbed hands before returning to Harry's side. Snape could see from here that the boy's pain had faded, that the potion had done its work, and he was now in a natural, easy slumber.
It was impossible...
Dumbledore's aged hand stroked across Harry's sweat-soaked brow, smoothing back the mop of wet hair, lingering upon the boy's scar. At long last, he looked up.
Snape stood there, gaping at him.
"Come now, Severus, with a mind as sharp as yours, surely you understand why this worked?"
Snape looked at Dumbledore, then he looked at the boy. No, no he didn't. He could think of how-- but no, that was impossible.
Dumbledore's eyes were still locked on him, strangely sad.
"Look at me, Severus."
Snape knew he had to follow Dumbledore's orders. He looked at him. Straight into those mournful blue eyes.
He barely heard Dumbledore's whispered, "Obliviate."
Snape stared ahead of him in disbelief as the memory restorative draught slowly released him; he recalled Dumbledore's voice telling him to resume his previous activities. And he remembered the cloudy walk back to his chambers, still in the grip of Dumbledore's spell. He'd looked up from grading papers, surprised at where the night had gone, wondering if the little brats had written such mind-numbing essays they'd actually put him to sleep for several hours.
Now he lurched to his feet, reeling. Dumbledore had obliviated him. Dumbledore had bloody obliviated him!
And he grew still.
Blood. His had cured Potter. That particular counter-poison-- only the immediate family was sufficient for the biological component. In rare cases uncles or cousins, but only very rarely. He would have to be--
Snape glared at one of his jars, as though willing it to explode. The only possibility--
It couldn't be. It just couldn't be.
Ah, but it could, a voice reminded him, thoughts flickering to that night, to those green eyes and that warm, salty skin...
Oh, no... Oh, please, no.
It was then Snape knew he was Harry Potter's father.