AP Chem brought us together. We went shoulder to shoulder amid all that fire and smoke and mystery.

It was fate, Diane would later say.

Two days before she arrived at Lanister High, Benjy Dunphy, my lab partner, got suspended for throwing a fist-size chunk of potassium into a sink full of water, shattering the sink and setting a girl’s hair on fire. I never liked Benjy, who had a hundred jokes about stopcocks and a hundred ideas of how to use science to rip girls’ dresses off or maybe make their bras fall open.

“Kit,” Ms. Steen said, “meet your new comrade in arms.”

Diane took her place beside me at the lab bench in her Peter Pan collar and smooth headband and an expression I remembered well from camp: focus, intensity, a wriggling vein at the temple. The tidiness of her notebooks, the bright and crisply cornered textbook cover of Chemistry in Action and Reaction!, it all spoke to me of seriousness and purpose.

Up front, Ms. Steen was giving titration instructions.

“I don’t know how I ended up in AP,” I said, trying to find a piece of paper in my notebook, stained with my mom’s coffee from the jolting drive to school. “I should have taken the rocks class. But Ms. Steen made me.” I rolled my eyes. “She believes in me.”

Diane looked at me but said nothing.

Then she pulled two pages from her spiral notebook and handed them to me, their edges neatly shorn. “Use this.”

Side by side, we moved through the titration lab, using the juice from two fat cabbages that Benjy surely would have held in front of his chest like big purple tits and I probably would have laughed.

Hair pulled back into a glossy barrette, Diane began labeling everything meticulously: Dr Pepper, Rockstar Energy Drink, tap water, Febreze, hand sanitizer, milk. I poured the cabbage juice into a watch glass and then into our sample cups and I was careful too.

Leaning in close for each reaction, I called out the colors: “Coke, orange—acid! Febreze, clear—base!” Diane recorded them with her little mechanical pencil.

When it was over, she wrote out the report on the spot in her even, exact handwriting, her pencil swirling and equations appearing.

“You can take that home and do it,” I said. “I mean, we can. It’s homework.”

“That’s okay,” she said. “I’ve done this lab before.”

“At Sacred Heart?”

“No, before that,” she said. Later, she admitted she’d mastered it for her fifth-grade science fair. I wondered how it felt being here with all of us. With me.

When the bell rang, she was still washing a burette with a long brush.

“That’s fine, Diane,” I said. “That’s plenty good enough.”

“I want to do everything right,” she said, shaking the burette dry. “I don’t want to make mistakes.”

By the end of the first week, Ms. Steen took us aside after class to compliment us on our lab reports and our A+ quiz scores.

“You’re going to be good for each other,” she said, smiling. My face went warm. A+.

“Maybe we should study together sometime,” I said to Diane as we walked out.

“Maybe. Sometime,” she said. If your dad dies, I thought, maybe even studying with a classmate takes you out of mourning or something. But even before, even at cross-country camp, I’d never known anyone so private. It felt like you could hurt her just by looking at her, or you could never hurt her at all.

Later that day, I spotted her leaving the school library, backpack heavy with books. I found myself following her.

You always wonder, when someone is trying so hard, what it’s really about. Whenever my dad started calling more or took me out for a surprise dinner at Benihana or sent me leopard-pom booties for Christmas as if I were twelve (or a stripper), it was always because he’d lost his job because his boss was jealous of him or he’d gotten married again to a woman he’d met at the OTB and, anyway, there would be no monthly support checks forthcoming, and Maybe you could even talk to your mom about floating me a nickel? How ’bout a hundo?

Maybe, for Diane, working really hard was a way of crouching low in her grief, of staying under the radar. Of hiding.

But in other ways she was impossible to miss. In class, her hand was up all the time. It was something to see, the way her mind wrapped around things. The way she’d always ask questions (“I’m just wondering why the chapter doesn’t mention stem-cell research even once?”). Sometimes, she even disagreed with something in the textbook or something Ms. Steen or Ms. Cameron, the English teacher, had said. I’d never see anyone do that before.

I wondered what it was like to care so much about ideas from books and to think about things like why we dream and if female brains are different from male ones. But maybe I cared too, because sometimes I found myself wondering about those things. I just didn’t show it. It was high school; you didn’t show things.

I followed her down the hall, only stopping when she did, drawn by a glossy flyer on the bulletin board by the counseling office.

Later, I’d remember the way she summoned me over, like she’d known I’d been following her all along.

“Kit,” she said, her fingertips light on my arm, “look.”

The flyer, stern-looking with a vaguely Germanic font, announced the application deadline for the DR. LENA SEVERIN STEM SCHOLARSHIP FOR WOMEN IN SCIENCES.

“The winner gets a full ride to State,” she said. “Tuition plus room and board. A stipend too.”

“Severin,” I said. “I saw a Dr. Severin at STEM camp over the summer.”

“It’s the same Lena Severin.” She said the name as though she liked how it felt in her mouth, and there was something about the way she pressed her fingers against the poster, like it was a piece of fine-spun silk. “I saw her talk once at the science museum. It meant a lot to me.”

“Me too,” I said, only realizing it now, after Diane said it. There had been something so exhilarating about it, the first woman scientist I’d ever seen and still the smartest.

It was just a few months ago, at the summer science program Ms. Steen had talked me into, even though it was embarrassing having to show my mom’s paycheck stubs to get in free. Six weeks of me racing between the Golden Fry and the lab at City Tech, injecting samples into the gas chromatograph while smelling hotly of chicken grease.

Most of the time, we were just doing grunt work—counting, washing and spinning cells, injecting those samples—but sometimes we had lectures from visiting big-time researchers who explained how they were going to change the world through polymers or a nanotech moisturizer that would speed the healing of diabetic skin wounds.

One was a biochemist, an MD/PhD who wore the highest heels I’d ever seen in real life, shaped like black cones. Her name was Dr. Severin, and with a name like that, you could never be a soft wisp of a girl. No, you were destined to be this.

At first, all I could do was stare at the pictures projected behind her, which were brain scans but looked like something you might see in astronomy or your most deeply felt, late-into-the-night dreams. But then Dr. Severin started talking about the female brain before, during, and after menstruation, which made at least one boy burst into nervous laughter.

I was the only girl in our school’s group of seven STEM scholars, and the boys seemed glad the room was so dark because Dr. Severin’s talk, as scientific as luteal phase and increased amygdala response sounded, seemed to speak to them of all the blood-horrors of being female. I don’t know how many of them had ventured into girl bodies or how deeply, but they had the look of small boys expecting to find teeth between every woman’s legs.

“And isn’t it interesting—significant, even,” Dr. Severin said, looking at the squirmiest guy, the red-faced, white-lipped one down front, “that menstruation is a source of so much horror in our culture?”

The boy nodded grimly, as if conceding something.

“After all,” she added, “to quote Deuteronomy, The blood is the life.”

Which is a line I remembered from Dracula movies.

I loved hearing her, not just because she was a woman scientist and young with a skunk streak in her dark hair and earrings that were little silver scythes, but because she talked with such fervor about her mission: to unravel the enigma—“and plague, really”—of PMS and something called PMDD, which was even worse. In the old days, if you had it bad, they used to just cut out your ovaries to “cure” you. “Snip-snip-snip,” she said, making a little scissoring motion with her hand.

“But how far are we, really, from that?” she asked. “There are still those who deny that these conditions exist. Medical students are rarely trained in diagnosing or treating them. When women come for help, they’re frequently dismissed with a roll of the eyes. Worst of all, research is scant and, frankly, sad. I aim to change that, to stake a claim for the health of women caught in these unbearable snares, prisoners of their own bodies.”

Up on the screen, the last scan hovered, ghostlike. The brain of a woman with severe PMDD. I couldn’t stop staring at its shape, like a strange mushroom you’d find in the woods. All its shadows and hollows—you could almost see a face in it, eyes like caves and a mouth open like a scream.

“So think about that when you decide your own path in the sciences. Think about where you want to go, what dark terrain you want to uncover. The mind, the body, the complicated junction between the two—it’s dangerous stuff. It’s thrilling stuff. What you do will matter.”

When she finished, slapping her laptop shut, a cluster of Severinites, graduate students in versions of her oversize glasses and dustless black attire, followed her from the lecture hall, their dark bodies moving like a snake’s winding tail.

For days after, I imagined myself as one of those chic Severinites, trailing her, joining in her grand mission. By the end of the summer, I’d decided to change my senior-year schedule like Ms. Castro had wanted me to, enrolling in AP Chem instead of accounting, physics instead of communications.

“She was my favorite,” I said to Diane now. “Her whole lecture was about periods.”

“Yes,” Diane said, then, lowering her voice, she said with a sneaky look, “All that blood talk.”

“The blood is the life,” I said, grinning. “The blood is the life.”

Diane smiled widely, the first time I’d ever seen it. A dazzling, even-toothed smile with something wicked in it.

“The boys all looked like they were going to throw up,” she said, covering her mouth, hiding that smile.

From the counseling desk, Mrs. Kreuzer shot us a snippy look, but Diane didn’t see it, her eyes back on the flyer.

“It’s for girls. Just for girls,” she said, palm flat on the glossy paper. “I’m going to apply. You should apply too.”

“But you have to be in the top two percent of your class in science,” I said.

“Yes,” she said. “That’s right.”

I can’t pretend I didn’t feel a rush of something come upon me then. Could I? I thought. Because if Diane Fleming thought I could…

“Maybe I’ll apply,” I said, surprising myself.

Diane nodded, as if to say of course I would, and turned back to the flyer, her lips faintly moving as she read the requirements again, committing them to memory.

That night, I ran the numbers on my GPA. I’d have to get perfect grades in my three science courses the first and second quarters. It didn’t seem possible. Except maybe it was.

Before Diane, I mostly just focused on the math of it. I was always good at that, balancing my mom’s checkbook and working the register at the Golden Fry. I guessed I’d be a CPA, maybe, or work at a bank. But lately I’d started to feel like there was more there, hovering. Maybe it was the Severin application. It gave everything we were doing a center, a unifying force. It wasn’t about the grades, but what the grades could get you. Where they could get you. Places no one in Lanister, or at least my corner of it, had ever been.

I asked Ms. Castro at the counseling office about it.

“This is smart, Kit. I’ve been telling you, you can’t apply only to City Tech,” Ms. Castro said, reading glasses low on her sloping nose. “You have to apply to State. Your grades are competitive. Your test scores are strong.”

But in my quieter moments, standing over the Henny Penny fryer at work, my skin feeling tight and crisp as the staggered chickens’, roiling, all I could think of was the students I’d seen at STEM camp. The way they spoke about DNA nanotechnology and super-resolution microscopy, the paper-thin laptops and tablets they carried, and the group of Severinites trailing their chic idol through the sleek auditorium.

“You should really do it, Kit,” Diane whispered to me in AP Chem the next day. “Who in here’s smarter than you?”

You mean other than you? I thought. But there was a stirring in me, a flutter of sparrows in my chest.