Obliviate!
Luna Lovegood slowly walked towards the Headmaster's Office. The smile on her face - that smile that made it seem as if she knew a secret no one else did, wouldn't leave her face, even when she realised how great the risks were.
Looking around, making sure no one was there, she whispered the password to the gargoyles: Dumbledore.
"Come in."
Luna entered the round Office, not without admiring Dumbledore's taste. And the fact that Snape hadn't changed anything in here.
"Headmaster Snape." She pulled out her wand and put it on the table before her. In Snape's reach.
With a small nod, she allowed him to take it, but the headmaster didn't move. Handing over your weapon was a sign of trust. Snape knew that and his eyes showed confusion before he managed to retrieve his expressionless mask.
"Headmaster, I require your help."
Without being asked, she sat down.
"You have two choices..." she started and cleared her throat. "...either you teach me how to perform occlumency or I have to be obliviated."
Snape looked weary. He had absolutely no clue what this girl was talking about, but if she was anything like her father it would be about Crumple-Horned Snorkacks or Nargles...
"Miss Lovegood, I have no time for your silly games. I will not..."
She put a finger on her lip to stop his rant and the sheer cheek of this gesture made him fall silent.
"Headmaster, my father and I are in danger. The Quibbler now openly takes sides..."
Snape leaned back and regarded the strange blond girl.
"I cannot help you", he said finally, unable to remove the small hint of regret from his voice.
Luna smiled. "Yes, headmaster, I am quite aware of that. However, if I ever get captured... there is a possibility that I will spill out your secrets to You-Know-Who himself."
Snape's mouth nearly fell open and he quickly made a move for Luna's wand.
She grabbed his hand and stared at him... as if searching for something. Her hand was rested on his and she concentrated hard to transfer all sympathy and comfort she felt for the potions master into that touch.
"Lily Evans..." she started, without taking her big blue eyes of him... "invented the greasy hair hex. She used it on you. Maybe when she was angry. You never removed it. You must have loved her very much. I am very sorry that she hasn't left something better for you to remember her."
Snape's breath escaped with a hissing noise and he broke the eye contact to stare at Luna's hand.
"Headmaster, I have spent a lot of time in the forbidden forest last year. I overheard a few conversations between you and Professor Dumbledore. I am very sorry you were the one who had to..."
Snape jumped up, pointing Luna's wand at her chest. Luna flinched a little when she heard the heavy chair crash on the floor. It was a nice chair.
"How much do you know?" his voice sounded more harsh and threatening than he had intended it to be.
Luna raised from her seat, her hair streaming out behind her. She still showed no fear. But trust.
"I know that you have been protecting us. I know that you have made sure we weren't exposed to the Carrows more than necessary. I know you have no allies. I know I admire you."
Snape suddenly looked very tired. And old.
"There is no need for that." What was supposed to be sarcasm came out as a pathetic whisper.
"Rather the opposite, headmaster. Almost everyone is burdened with grave mistakes from the past which lead to someone being harmed."
Snape winced and turned away before she continued.
"...but after all these years... you still take it that hard. I think that is rare. And rather admirable."
Luna studied her favourite teacher for a long time, searching. She wanted to know if her words had arrived.
Snape was breathless. And speechless for that matter.
She lent forward, begging him to maintain the eye-contact and continued, fearless.
"She would have been very grateful you know. You have done a very good job protecting Harry."
Snape swallowed hard. He couldn't believe the courage and wisdom that girl possessed. Not reckless, thoughtless Gryffindor courage, but rather clever and calculating.
"I cannot teach occlumency, Miss Lovegood. The risks are too high. Unfortunately I am forced to choose the second option."
Luna shook her head slightly. "It's not a very nice job, is it." Her voice was full of regret. "But it has to be done."
Then she considered him with the most earnest, appreciative smile he had ever seen. "Thank you for everything headmaster."
After a second of hesitation... No one had smiled at him for months, he raised Luna's wand again.
"Obliviate!" he whispered and followed it up with a louder:
"CONFUNDO!".
"Miss Lovegood. You remember not one conversation between Dumbledore and myself in the forest. You cannot trust me. You are wrong." He rounded the spell up with a silent "Ennervate."
Luna blinked, confused. The smile was wiped from her face and quickly replaced by sadness and disappointment. Her large blue eyes filled with tears.
Snape pulled himself together, smirked and handed her wand over to her. She had been very clever. Knowing that the Dark Lord would check all the spells that were cast from his wand, she had offered him her own.
"Fifty points from Ravenclaw for disobedience. Dismissed."
"Headmaster... I don't trust you anymore..."
Snape winced a little and tried hard to force some sarcasm into his voice.
"Oh well, Lovegood. That's my belief system destroyed. I suppose I should go to bed and cry my eyes out."
Luna turned around and ran off. Snape seemed to hear a muttered "maybe you should" but he wasn't sure.
She kept running, and didn't turn around, even when she heard loud thumping and things breaking in the headmaster's office.