This is set during Harry's fifth year, one of Dumbledore's schemes to kill Lord Voldemort.
Snape slowly made his way up to Dumbledore's office, dragging his feet as he went, he was wondering what the old fool had come up with this week, for a clever man his schemes really did seem awful.
He knocked on the door and didn't wait for a knock before pushing the door open and sitting in the seat in front of Dumbledore's desk. Dumbledore sat behind it, moving his fingers in an excellent impression of a mafia boss and smiling at Severus.
"Hello," Said Dumbledore brightly. "I presume you know why I have called you here."
"To get rid of Voldemort?" Snape said, a hint of annoyance in his voice. "Since that's all I'm good for nowadays."
"Oh, Severus, do not be so dramatic, I think you will find this scheme...how should I put this...more...er...interesting than the others."
Severus looked worried the plans of the past six months had not been exactly normal and he was worried what Dumbledore could find more interesting than the plans that he had come up with so far. Severus' favourite by far had been the one where Dumbledore had suggested they all dress up as women, Severus had never quite grasped how that plan would work and had a niggling suspicion that it could just have been because Dumbledore actually wanted to dress up as a woman.
Severus shuddered at the mental image, and focused again on Dumbledore's blue eyes.
"So," said Severus. "What is this oh so interesting plan?"
"Well," said Dumbledore, smiling widely. "You know that Voldemort admires himself greatly? And sees himself as the most powerful greatest wizard who has ever lived?"
"Yes," said Snape, getting more worried by the second. "And your point is?"
"Well," said Dumbledore. "I think that Voldemort would have trouble killing himself, even if it was just an image of himself, so if we use polyjuice potion to transform into an army of Voldemorts then maybe he will not be able to kill us, and we will have a lot more success in defeating him."
Snape sighed and thought to himself, well he supposed, it was a better plan normal. He thought it over and then his mind hit on a snag.
"But the dark lord does not have hair!"
"Not on his head, no,"
Severus looked ill.