Mia must be tired; otherwise she would never mention the prospect of leaving us—not within Annie’s hearing—even though the eventual day of parting is inevitable. Mia’s very presence is a miracle, one based on a generosity I can scarcely comprehend. Two years ago, when she was a superachieving high-school senior at my alma mater, Mia took care of Annie for one summer, then during the school year on afternoons when I was working. She was the perfect babysitter: a smart, vivacious, and motivated girl from a family of modest means, driven to work for the things her private school classmates took for granted. Her drive and practicality rubbed off on Annie every day, and I was thankful.

But late that year, a classmate of Mia’s drowned in a nearby creek, and a childhood friend of mine was charged with killing her. Mia proved instrumental to solving that murder, and as a reward, my grateful friend—a physician—helped her attain something that had proved beyond her reach, no matter how hard she worked: tuition money for her first-choice college, Harvard.

By pure chance, Mia was on her way home for Christmas break when Caitlin was killed. As soon as she heard the news, she came over and did all she could to comfort Annie, who had already begun to regress to the paralyzing, hyper-anxious state she’d experienced after her mother died in Houston. Within a week, Annie developed a worrisome dependence on Mia. I didn’t know how I was going to keep her from losing control when Mia had to return to Massachusetts. To my amazement, though, three days before Mia was scheduled to leave, she sat me down and said she’d decided to take a semester off in order to help Annie “get back to normal.”

I argued, but not too hard or for too long. Mia told me she’d been scheduled to do a semester at an archaeological dig in the Yucatán, so it wasn’t like ditching a real semester. By this time, my mother had already decided to relocate to be near Dad’s prison, and that settled the issue.

“There’s a light,” Mia says. “Up on the left.”

She’s right. What I remember as a solitary service station stands on the edge of the flat fields about a mile ahead, like some radio relay station in the desert. Taking out my cell phone, I speed-dial Tim behind us.

“What’s up?” he asks.

“We’re pulling off at that gas station. Girls need a bathroom.”

“Let me catch up before you turn.”

“Copy that.”

This kind of tactical conversation has become second nature over the past weeks. Sixty seconds’ driving carries us to the turn, and Tim is behind us by the time I roll the wheel left and clunk onto a gravel-studded dirt lot near the concrete pad that supports the old gas station.