“What makes her dangerous though,” Mrs. Johnson says, “is her intelligence. She has a genius-level IQ on testing. Did you know that?”

“I… I’m not surprised.”

“A math genius.” I see a twinge of pride for the first time. “If she could focus, I bet she could win a Nobel Prize. But… well, that’s out of the question now.”

There’s no Nobel Prize in math—a fact I know thanks to Sam. Instead, there’s a Field’s Medal, which is only given every four years and rarely given to mathematicians over the age of forty. Sam is realistic about his chances of winning one, especially now that he’s thirty-eight, although he admits he was never a true contender. I think my Field’s Medal is out the window, he sometimes jokes.

“You said she’s dangerous.” My heart speeds up in my chest. “Dangerous in what way? She seems perfectly normal.”

“Oh, she’s good at playing the part.” She lets out a joyless laugh. “But don’t be fooled. My husband and I started locking our doors at night, if you know what I mean.”

I stare at her. “You did?”

“Oh yes.” She stares off into the distance. “I knew she had problems but I never thought she was dangerous until her sophomore year of high school. She and her best friend Sandy were fighting over the same boy. Silly stuff, you know? But girls are so emotional at that age, and they had a falling out, and then…”

I get a horrible sinking feeling in my chest. I don’t know if I want to hear the end of this story, but how can I not hear it? “Then what?”

She shuts her eyes for a moment. “Sandy went missing.”

I squeeze my knees so tightly, my fingers hurt. I can’t believe I invited this crazy person into my life. How could I have been so stupid? “Maybe she just ran away? Girls do that.”

“No, she didn’t run away.” Mrs. Johnson’s eyes grow distant, staring off into nothing. “They found her floating in the Charles River a week later.”

I clasp my hand over my mouth. I think I’m going to be ill. I really do. “Mrs. Johnson, can you… can you tell me where the bathroom is?”

She points a long, skeletal finger down the hallway, and I grab my purse and run. I make it to the toilet in time, but all I can manage is a dry heave. I skipped lunch because I was so anxious about my appointment with Frisch, so there’s nothing in my stomach.

My head spins as I straighten up and look in the mirror. My face is deathly pale and my black hair is disheveled. I run my fingers through my hair and splash water on my face, but none of it helps. I consider freshening up my makeup, but what’s the point?