Once, in a neurobiology class in college, our professor showed us an MRI of a man’s brain that was covered with white blobs. He asked us what we thought they were. Everyone guessed tumors. I was the only one who got it right.

“Tapeworms,” I said. I’d seen them countless times in my mom’s clinic, in the slurry excrement of the strays she brought home. They start out as ribbons, unwinding to a dozen, two dozen feet, loping around the intestine. But before that, they’re larvae and they can drift effortlessly into the bloodstream, find their way to the brain. Burrowing in. Getting trapped in its cavities, sprouting like grapes. They can thrive there, live there for years. Pressing against the membranes, swelling the pinky-gray tissue like a tissue-paper pom-pom.

If I sliced open Diane’s beautiful, extraordinary brain, I feel certain of what I’d see: a swarm of worms, a cluster of sickly grapes, pushing against the chambers of her brain, inflaming it.

She didn’t ask for it, but it’s there. Maybe she can’t help it, a fatal combination of nature and nurture, a derelict upbringing, a feckless parent.

Maybe she can’t help it, but can’t we all say the same?

We’re in Severin’s office, waiting for our turn. Diane will take a long, long time, I think. She’s been waiting quite a while to talk. Maybe forever.

Severin brings us both coffee from her secretary’s machine and slugged with a shot of brandy from her file-cabinet drawer.

“I can’t decide about you, Kit.”

The voice, like smoke-heavy curtains rustling behind you.

“Can’t decide what?” I ask. The lights so low. The blinds half shut as in some lost film noir. The room is gray, or white, or no color at all.

“Whether you’re the luckiest or unluckiest person in the world.”

She shakes a cigarette loose from a small pine box shaped like a miniature coffin on her desk.

“Let me know if you figure it out,” I say, rubbing my neck.

Cigarette in mouth, she feels for her lab-coat pocket on instinct, but she’s still wearing her party costume, the sleek leather dress of a winner.

“Don’t suppose you have a light?” she asks, maybe smiling a little.

I shake my head: Not this time.

She opens her desk drawer and fumbles around until she finds a box of safety matches. She lights up and takes a drag, leaning back in her chair.

“Oh, Serge,” she says, cigarette dancing between her lips. “So he just tucked the body away, eh?”

“Yes. He said he couldn’t finish the job. The job was for you?”

She nods slowly. “He told me he’d taken care of it. The campus incinerator. It took eighty-five minutes, he told me. He lied.”

“So he’s the one who told you about Alex? All this time, you knew?”

“Certainly,” she says, surprised. “It’s my lab.”

“But Serge got it all wrong,” I say. “He thought Diane killed Alex.”

Dr. Severin looks at me, blinking twice. “Didn’t she?”

Her face impassive, she nods slowly as I tell her what really happened.

“And you let him run the flash column even with the crack in the glass?” she asks once, then twice.

“I told him to stop,” I say. “Over and over.”

The look she gives me is one I will feel for a long time. It’s a cold thing under my skin.

“You know how he was,” I say. “He was never careful. He didn’t have to be.”

She squints. “Maybe he did.”

“What about you?” I counter. “Look what you did. Look what you made Serge do.”

“I’m not proud of it,” she says, her voice clipped. “Who could be proud of any of this?” She sighs, looking at her hands, which I now see are shaking. “I loved them both.”

I look at her, startled. “Why, then? To protect yourself? The lab? The work?”

“Don’t moralize with me, Owens. Not now. None of us is off the hook.”

“I didn’t—”

“There’s a reason, after all, you feel so comfortable in that vivarium,” she says. “With the mice, and the rats.”

Rats. The word stings, rings in my ears.

“So you know everything,” I say. I take a breath, thinking of sitting across from Ms. Castro all those years ago. “I should’ve gone to the police.”

“But you didn’t, did you?” she says. “Clever girl.”

I reach for the pine box, take a cigarette.

“Well, everyone has her weaknesses,” I say. “Is Diane yours? First she’s your case study. Then she’s your intern. And once again you found her.”

“If you prefer to phrase it so romantically.”

“Are you lovers?” I ask, taking a long drag, my first-ever cigarette. It smells like my dad.

Severin crosses her legs, black leather dress like an oil slick catching the light.

“Oh, Kit,” she says, smoke swarming us both, “you are still so young.”