When I was around 5 or 6—honestly, I can’t remember exactly—I spent almost every day with my best friends: the twins who lived next door. We were inseparable, the kind of friends who did everything together, whether it was running barefoot through the grass or daring each other to eat wildflowers. If they jumped, I jumped. If they climbed trees, so did I. They always used to wear their hair in pigtails. I even copied that. Everyone could see that we were really close.

One warm afternoon, as we wandered around the neighbourhood as usual, I spotted the big house down the street. We’d talked about it many times, imagining all sorts of mysteries. We...