I had always hated math.

Equations, formulas, the way numbers refused to make sense no matter how long I stared at them—it was the one subject I could never fake my way through. Most days I sat in the back, half-asleep, counting the minutes until the bell. That changed the Monday Mr. Harlan walked into the room.

He was impossible to ignore.

Tall, broad-shouldered, dark hair cut short, jaw sharp enough to look severe. He wore a simple white shirt with the sleeves rolled up to his elbows and dark trousers that fit too well for a teacher. When he set his bag on the desk and looked over the...