The door vibrated as another sound buzzed through Clarke’s ear. Her breath caught in her throat. That’s impossible. But when the sound came again, it was even clearer.

It wasn’t just a scream of anguish. It was a word.

“Please.”

Clarke’s fingers flew over the keypad as she entered the first thing that came to her head: Pangea. It was the code her mother used for her protected files. The screen beeped and an error message appeared. Next she entered Elysium, the name of the mythical underground city where, according to bedtime stories parents told their children, humans took refuge after the Cataclysm. Another error. Clarke tore through her memory, searching for words she’d filed away. Her fingers hovered above the keypad. Lucy. The name of the oldest hominid remains Earthborn archaeologists ever discovered. There was a series of low beeps, and the door slid open.

The lab was much bigger than she’d imagined, larger than their entire flat, and filled with rows of narrow beds like in the hospital.

Clarke’s eyes widened as they darted from one bed to another. Each contained a child. Most of the kids were lying there asleep, hooked up to various vital monitors and IV stands, though a few were propped up by pillows, fiddling with tablets in their laps. One little girl, hardly older than a toddler, sat on the floor next to her bed, playing with a ratty stuffed bear as clear liquid dripped from an IV bag into her arm.

Clarke’s brain raced for an explanation. These had to be sick children who required round-the-clock care. Maybe they were suffering from some rare disease that only her motherof y her m knew how to cure, or perhaps her father was close to inventing a new treatment and needed twenty-four-hour access. They must’ve known that Clarke would be curious, but since the illness was probably contagious, they’d lied to Clarke to keep her safe.

The same cry that Clarke had heard from the flat came again, this time much louder. She followed it to a bed on the other side of the lab.

A girl her own age—one of the oldest in the room, Clarke realized—was lying on her back, dark-blond hair fanned out on the pillow around her heart-shaped face. For a moment, she just stared at Clarke.

“Please,” she said. Her voice trembled. “Help me.”

Clarke glanced at the label on the girl’s vital monitor. SUBJECT 121. “What’s your name?” she asked.

“Lilly.”

Clarke stood there awkwardly, but when Lilly scooted back on her pillows, Clarke lowered herself to sit on the bed next to her. She’d just started her medical training and hadn’t interacted with patients yet, but she knew one of the most important parts of being a doctor was bedside manner. “I’m sure you’ll get to go home soon,” she offered. “Once you’re feeling better.”

The girl pulled her knees to her chest and buried her head, saying something too muffled for Clarke to make out.

“What was that?” she asked. She glanced over her shoulder, wondering why there wasn’t a nurse or a medical apprentice covering for her parents. If something happened to one of the kids, there’d be no one to help them.

The girl raised her head but looked away from Clarke. She chewed her lip as the tears in her eyes receded, leaving a haunting emptiness in their wake.

When she finally spoke, it was in a whisper. “No one ever gets better.”

Clarke suppressed a shudder. Diseases were rare on the ship; there hadn’t been any epidemics since the last outbreak they’d quarantined on Walden. Clarke looked around the lab for something to indicate what her parents were treating, and her eyes settled on an enormous screen on the far wall. Data flashed across it, forming a large graph. Subject 32. Age 7. Day 189. 3.4 Gy. Red count. White count. Respiration. Subject 33. Age 11. Day 298. 6 Gy. Red count. White count. Respiration.

At first Clarke thought nothing of the data. It made perfect sense for her parents to monitor the vitals of the sick children in their care. Except that Gy had nothing to do with vital signs. A Gray was a measure of radiation, a fact she well knew as her parents had been investigating the effects of radiation exposure for years, part of the ongoing task to determine when it’d be safe for humans to return to Earth.

Clarke’s gaze settled on Lilly’s pale face as a chilling realization slithered out of a dark place in the back of Clarke’s mind. She tried to force it back, but it coiled around her denial, suffocating all thoughts except a truth so horrifying, she almost gagged.

Her parents’ research was no longer limited to cell cultures. They’d moved onto human trials.

Her mother and father weren’t curing these children. They were killing them.

They’d landed in some kind of clearing, an L-shaped space surrounded by trees.

There weren’t many serious injuries, but there were enough to keep Clarke busy. For nearly an hour, she used torn jacket sleeves and pant legs as makeshift tourniquets, and ordered the few people with broken bones to lie still until she found a way to fashion splints. Their supplies were scattered across the grass, but although she’d sent multiple people to search for the medicine chest, it hadn’t been recovered.

The battered dropship was at the short end of the clearing, and for the first fifteen minutes, the passengers had clustered around the smoldering wreckage, too scared and stunned to move more than a few shaky steps. But now they’d started milling around. Clarke hadn’t spotted Thalia, or Wells, either, although she wasn’t sure whether that made her more anxious or relieved. Maybe he was off with Glass. Clarke hadn’t seen her on the dropship, but she had to be here somewhere.

“How does that feel?” Clarke asked, returning her attention to wrapping the swollen ankle of a pretty, wide-eyed girl with a frayed red ribbon in her dark hair.

“Better,” she said, wiping her nose with her hand, unintentionally smearing blood from the cut on her face. Clarke had to find real bandages and antiseptic. They were all being exposed to germs their bodies had never encountered, and the risk of infection was high.

“I’ll be right back.” Clarke flashed her a quick smile and rose to her feet. If the medicine chest wasn’t in the clearing, that meant it was probably still in the dropship. She hurried back to the still-smoking wreck, walking around the perimeter as she searched for the safest way to get back inside. Clarke reached the back of the ship, which was just a few meters from the tree line. She shivered. The trees grew so closely together on this side of the clearing, their leaves blocked most of the light, casting intricate shadows on the ground that scattered when the wind blew.

Her eyes narrowed as they focused on something that didn’t move. It wasn’t a shadow.

A girl was lying on the ground, nestled against the roots of a tree. She must have been thrown out of the back of the dropship during the landing. Clarke lurched forward, and felt a sob form in her throat as she recognized the girl’s short, curly hair and the smattering of freckles on the bridge of her nose. Thalia.

Clarke hurried over and knelt beside her. Blood was gushing from a wound on the side of her ribs, staining the grass beneath her dark red, as if the earth itself were bleeding. Thalia was breathing, but her gasps were labored and shallow. “It’s going to be okay,” Clarke whispered, grabbing on to her friend’s limp hand as the wind rustled above them. “I swear, Thalia, it’s all going to be okay.” It sounded more like a prayer than a reassurance, although she wasn’t sure who she was praying to. Humans had abandoned Earth during its darkest hour. It wouldn’t care how many died trying to return.