Chapter 1: The Wager
McGonagall was on the prowl. More accurately, she was hunting a specific Potion's Master who had returned alive for the school year, to the shock of all. Without You-Know-Who or Dumbledore on his back, he was an annoyingly competent instructor. He was the talk of the school for that fact alone, then add to it his hero status and rumoured-invincibility to snake venom and Snape was really grating on her nerves.
It would have been fine, however they had been unable to find a Potions or Transfiguration teacher post-war, and so they had agreed to be co-Headmaster/Headmistress. And the horribly popular man had been conniving behind her back and giving his Slytherins private Potions instruction to boot. She was putting an end to it.
She skittered up behind the herb-scented man, knowing she couldn't surprise him, but determined to try anyways. To her surprise, the potion master dropped to his knees and transformed, his body shifting in what she knew was an Animagus transformation. After a moment, his three-headed runespoor for rounded on her with a hiss.
Without even pausing, McGonagall shifted back to human.
"Severus, when on the God's green earth did you become an Animagus?" McGonagall demanded.
Snape easily joined his colleague on his feet, giving her an award-winning smirk. "Feeling threatened, Minerva?"
"Pfft, hardly," she rejected quickly. "I am merely curious."
"Alright. It was before the Second War," he answered. "It was meant to give me an edge, should the Dark Lord ever figure me out. No one knew about it, not even Dumbledore."
"You've been hiding it all this time?!" McGonagall screeched at him.
"Obviously," Severus quirked his brow at the entertainment he'd been given. "Now, Minerva, was there something you wanted? I've been rather busy, and I'd like to get a good night's rest."
"Perhaps you wouldn't be so busy if you weren't giving extra tutoring to your Slytherin Seventh-Years!" McGonagall accused.
Snape had been waiting for this. All he needed was the right moment, the right inflection of his tone, and he would have a highly entertaining rest of term. It was so easy to get the strict lady to bow into a little bit of avarice and take a gamble with him, but the conditions had to be just right.
"And should I not aid my students, Minerva?" He quirked his eyebrow, knowing it'd infuriate her more. "Seems against your Gryffindor-ish tendencies."
"Stop making this about Houses, Severus!" she demanded hotly. "If you're providing extra instruction, you need to offer it to all the students, not simply those from your own House!"
"I find my snakes far more talented in their subjects than your lazy little lions," Snape jeered her. "In fact, I'm so impressed with them I'm going to begin them in the process of becoming Animagi."
"Are you mad?" McGonagall shrieked. "Months of work, just to prove your house better?"
"If you insist, your Gryffindors may join," Snape goaded easily. "However, I'm unsure as to whether any of them will make the transformation."
"Circe's tit they will!" McGonagall declared. "You provide the potions and the mandrake leaves, and we'll both provide our house with instruction. So help me, Gryffindor will have more transformations that your house by the end of the year!"
"The end of the year provides the failures with too much time," Snape deflected easily. "It takes them 30 days of holding the mandrake leaf in their mouths and saying the incantation, then an additional 2 for the potion to be prepared, and AT MOST another week to transform. 39 days, Minerva. Whoever has the most Animagi in their house wins the Headmastership."
And so a Wizard's bet was born.