A Trade-Off

There was once a girl who was best friends with a boy named Tom. She was a Gryfinndor and he was a Slytherin, but they made it work. Their friendship was an accepted fact of life at Hogwarts School of Witchcraft and Wizardry. The headmaster was an idiot, Binns' class was boring, Evan and Tom were rarely seen without the other. What happens when events beyond their control tear them apart?

The answer: "I will find you. I promise."