Disclaimer: Will I ever own Harry Potter, or more importantly - Severus Snape? No? I thought not.

Author's Note: I don't know where this story came from. I really don't know.

Summary/ Story line: There is a traitor among the Hogwarts students. But they don't know who. Snape goes under cover and Sirius teaches potions...

Undercover

Chapter One: That was low!

"Severus, my boy. I have a very important mission for you," Dumbledore smiled.

"What is it headmaster? What do you need of me?"

"I need you to become my spy, Severus."

"Headmaster, I'm already your spy." Snape was confused. 'The senile old coot has finally lost it' he thought.

"No, no. Not spying on Voldemort, I want you to spy on the students," Dumbledore's eyes were twinkling like crazy.

"You've lost me," Snape frowned.

"If only," Sirius Black snorted as he walked into the headmasters office.

Professor McGonagall looked disapproving.

"Good evening Sirius," Dumbledore greeted.

"Evening. Why's that git here?" he nodded towards Snape.

Snape threw him a dirty look.

"Because Severus has agreed to take on a students form again and seek out a possible traitor among the children," the headmaster stated, ignoring the insult to Snape.

"I have not agreed to that!"

"I'm afraid it's too late to back out, Severus, as you've already consumed the youthing potion."

"I haven't taken a youthing po-" he stopped abruptly as he began to shrink a few centimetres. The lines on his face smoothed out and his clothes loosened.

"I took liberty of slipping some potion into your gravy," Dumbledore smiled innocently.

"Headmaster, that... that was low!" Snape raged.

"Not unlike yourself, Snivellus." Sirius was laughing his head off. "Look... hahaha... you're like... hahaha... two feet shorter than me now..."

"Go to hell, flea bag!"

"Come on now boys. No need to fight. Sirius, obviously Severus will be unable to teach classes."

Sirius sniggered.

Dumbledore continued "So I want you to take over until his assignment is over."

Snape looked appalled. Sirius Black teaching his class? So what if the man had been proven innocent of murder. He's still an incompetent fool in Snape's eyes. "You can't be serious! He leave my classroom in rubble."

"He is qualified to teach potions." Sirius smirked. "even if he is not nearly as brilliant as you." The now young Snape puffed his chest with pride. "Are you up to the task, Severus?"

"I am indeed."

"Great. Now your new name will be Richard Daniels. Is that acceptable?"

"It is."

"Good. I will take you to the Gryffindor dormitory's."

"Pardon me. Gryffindor dormitory's?"

"Yes. You hardly expected to be in Slytherin?"

"I suppose not, but Gryffindor? Why not Ravenclaw?"

"The other houses have already been done with different methods."

"Fine," the now 17 year old snapped.

"I guess I'll be your teacher, Snivillus," Sirius smirked. (there's a lot of smirking going on, isn't there?;)

"Shut up, mongrel!"

"Sirius, Richard, you have a student teacher relationship now. You have no past history as Sirius and Severus."

They both glared at him.

Next chapter - the first potions class.

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