The morning starts off warm and humid, with hints of a thunderstorm lingering in the air. I carefully slip on my tail as usual, lowering myself into the pool before pausing at the gate.

Crystal hovers in the main enclosure, her skin almost glowing under the soft glare of sunlight. My body tingles as I sidle on up to her, about to throw my arms around her neck when she suddenly turns and grins.

It isn't Crystal. It is another mermaid with the same blonde hair and dark blue tail, but not Crystal. I already know what the arrival of this new mermaid means.

Crystal is not coming back.

I watch from the corner as she swims her laps, overcome with the urge to be sick. I'd thought after Muriel's disappearance that I'd never have to go through the pain of losing someone again; it seems I couldn't have been more wrong.

At some point, Asia and Jewel both swim up beside me. I am vaguely aware of one of them calling my name, but I can't work out who.

Finally, I turn to face them, their features distorted through the tears. "She's gone," I say. "Crystal is gone."

Jewel's face hardens as she stares at the new mermaid, and I know it is because if she doesn't get angry, she'll cry, instead. "We're all going to end up the same way, thanks to Crystal," she mutters, but her cheeks have already begun to redden with grief.

Asia's eyes narrow as she turns to Jewel. "Do you think Crystal was of sound mind when she did this?" she snaps. "It had to have been the darkness controlling her. We all know what it feels like."

I raise my eyebrows, surprised at the anger in her voice. She has always been the peacemaker, the one who keeps her cool, no matter what awful situation Marine World puts us into.

For once, Jewel looks apologetic for her outburst, though she still purses her lips. "I know," she says, her voice softer now, "but think about everything we've worked for. Getting these stupid trainers to trust us, performing every single trick perfectly, and for what? So that they can just discard us like we're nothing? Replace us with lookalikes?"

We drag our gazes over to the new mermaid, who stares back expectantly with a perfect, poised smile.

"Hi," she says, blinking once. "I'm Muriel."

My stomach drops, every muscle in my body stiffening. How could they? How could they get rid of Muriel and then name this new mermaid the same thing? Didn't Muriel mean anything to them? Don't we?

Jewel suddenly lunges for new Muriel, forcing me to grab her arms to keep her back. "It's not her fault," I hiss, but even I can't stop the hatred swirling in my lungs. "It's not her fault," I mutter again, this time for my own benefit.

"Ah, you've met."

I look up to see Jackson standing in the doorway, his thin arms folded and a small, knowing smile on his face.

Beside me, the girls continue to smile. We are perfectly trained, all of us. Despite the inner turmoil currently ripping through ours stomachs, we don't dare let them falter.

"Aura, this is going to be your new enclosure mate," Jackson says, indicating to the new mermaid. "For now, she'll stay in her own room but in a few days, we'll get her settled into yours."

I smile at the new Muriel, hating her already–my only reason that she isn't one of us. She isn't Crystal.

Once Jackson leaves, the girls and I rest our elbows against the ledge, our tails extended behind us. For a while it is silent as we gather our thoughts, trying to come to terms with our emotions before the guests arrive.

"Well, there's no sense hating her for something she has no control over," Asia says. "All we can do is make her feel included."

I turn to Jewel, not surprised in the least to see the conflict in her expression. Out of the three of us, Jewel is the least adaptable to change. Our routines, knowing what needs to be done every minute–it gives her a sense of control.

"Asia's right," I say, giving Jewel's arm a squeeze. "I'm not exactly thrilled she's here, either, but we have to support her. The last thing we want is another mermaid going off the rails."

Jewel's eyes narrow at the very thought. "But she hasn't even made any effort to talk to us," she says. "She's just swimming around and acting like the guests are already watching us. It's making me sick."

"Maybe it's her way of coping," I say. "Maybe she thinks it's safer to act like someone is always watching."

"Maybe she's a Marine World spy," Jewel mutters. "Maybe they put her in here to see if we need to be gotten rid of, too."

I raise an eyebrow. "Maybe we're being a little paranoid."

"The guests are arriving," Asia says, pushing herself away from the ledge.

Jewel glances over her shoulder. When she turns around, her smile is back in place as though it never left her lips. "Show time."

I spend the rest of the day interacting with the guests, sticking to my routine and acting like the Aura they expect me to be, all the while watching my new enclosure mate through cautious, sideway glances.

She must have been living in the facility this whole time, waiting to take our place the moment we stepped out of line. Marine World has always made it clear that if we don't perform the way we're supposed to, we are simply discarded, just like the real Muriel.

Muriel Two performs her routine perfectly, exactly how she was trained to in the facility, no doubt, and I wonder what it is she is thinking when she smiles and twirls, if she is really as happy as she's making out or if she's scared to be anything else. Anything real.

Her presence takes me back to my own first night in the enclosure, roughly a year ago. After living alone in my complex for so long, it was terrifying being thrown into the night enclosure with girls I had never met.

I later learned they'd kept us all at the same facility but in separate rooms until they were ready to move us. I was the second to last to arrive, and the others had already formed a sort of pecking order between the three of them, one I'd refused to adhere to.

Back then, the four of us were forced to share the one night enclosure, which was so small we could barely move without bumping into one another, leading to frequent aggression between us.

Jewel was the worst. She was aggressive and would do just about anything to hold the attention of the guests. The trainers adored her for it–it was clear she was their favorite–but as soon as the park closed and there was no one around to dote on her, she changed into someone else. She was rude and selfish and took treasures that didn't belong to her, hiding them in different parts of the enclosure so that we couldn't find them again.

As a result, she was the one I chose to lash out at whenever the darkness took control. I spent many nights imagining her as my enemy, as the Once-ler to my Lorax. It seemed the more time I spent obsessing over my hatred for Jewel, the less time I had to think about the thing that was really bothering me: my lack of control.

After a while, when the damage we inflicted on one another could no longer be hidden by makeup, the trainers decided to separate the four of us into two separate enclosures, with Muriel and I in one and Asia and Jewel in the other.

The four of us only ever came together during the daytime, but even then, with it just being Muriel and me, I still couldn't stand being confined. I didn't care that my duty was to make the guests happy. I didn't want to perform in exchange for Marine World's safety, in exchange for a cure, and I hated that I didn't have a choice.

I'd known what was required of me when I woke up in my enclosure, but I still tried to lash out at the people who had taken my freedom–even if I didn't quite understand they'd taken it. This new girl hasn't fought. Hasn't suffered. She has done nothing since her arrival but swim around obliviously, and it fills my chest with a hatred she doesn't deserve.

Anger rages inside of me as the day stretches on, desperate for some kind of release. It seems the arrival of this new mermaid has sparked something dangerous within me, and as I smile at the children pressed against the glass, I am filled with this overwhelming desire to hurt them.

To hurt them all.

At three, I take my break as usual, my stomach churning when Rob's head swivels around the door. It means I am not going back into the enclosure for the one-on-one experience this evening. I am going for an experiment. I head into the bathroom and pretend to wash my hands, needing a moment to gather some courage before the Testing facility.

I have felt nothing but exhaustion since Crystal killed Alison, though it doesn't look like it. My skin remains unblemished, free from the hardships currently plaguing my mind, and for once, I am thankful for my altered genetics. The last thing I need is for the trainers to start trying to feed me more pills.

Once I'm ready, Rob leads me through the tunnels again, but instead of taking a left at the end toward the Testing facility, we take a right. I anxiously trail behind him through door after door until finally, I am led into a dark, narrow corridor.

Fluorescent lights flicker on, revealing a white, spiral staircase at least several floors high. I slide my hand up the gold railing as we climb, something feeling familiar about this stairwell.

Once we get to the fourth floor, Rob holds his card to the sleek, black pad and the light flashes red with the word Code. He punches in the numbers 8482, not bothering to shield the pad from my curious gaze.

We step into a room that looks similar to the Testing facility, with the same glass submersion tanks lining the back wall. I expect Rob to lead me over to one, but he pulls me toward the corner of the room where a familiar, white cube sits.

Every muscle in my body tenses. It is a machine I have only ever been in twice, but both times had made me wish they'd put me in the tank, instead.

Before I have time to panic, Rob is ushering me through the cube's door and closing it behind me, causing my legs to suddenly weaken as though unable to hold my weight.

Slowly, I lower myself onto the bench, wishing I was back in the enclosure. A mechanical noise groans out through the vents. I brace myself as the temperature plunges, squeezing my eyes shut.

Every time I am forced into these conditions, I can't help but do exactly what Marine World tells us not to do; ask myself why. These tests are designed to strengthen our performances, to impress the guests, but what good comes from knowing how far I can hear or how cold I can get?

"Please," I croak once the air becomes bitter, my voice sounding hoarse. "Let me out."

A slight buzzing noise sounds out from the intercom, before Dr. Wells' voice rings through. "You're fine, Aura. Remember what I told you the last time. You can warm yourself up if you just focus on controlling your body temperature with the power of your mind."

"I can't," I whisper. "I can't. I'm too cold."

My breath comes out in clouds of smoke, the air so cold that goosebumps have spread across my arms. I try to think back to what Dr. Wells had told me the last time I was in here, but my mind is thick with fog.

"Just imagine a flame along your spinal cord," Dr. Wells soothes. "Feel the warmth slowly spreading through your limbs."

I do as she says, recalling the first time Teresa had lit a candle for me. It was my eleventh birthday, and Teresa had snuck in cupcake with a single candle on top. I'd watched the candle burn for hours, fascinated that something so beautiful could be so destructive.

I try to picture that same flame now, imagining how it would feel to have it flicker near my frozen skin. Warm, inviting, like the kind of embrace only Teresa could give.

Tears form as I try to remember her features, but I can't. My memories of her have faded over the years, her features so distorted it's like trying to recall a dream. Somewhere along the way, without me realizing, my mind has become like the nets they use to capture us in the water – intricately designed with too many holes to count.

A small spark ignites, a glimmer of warmth in the tip of my fingers that makes them twitch. Surprised, I focus harder, imagining the fire spreading through the rest of my limbs until I am consumed in its flames.

"You're doing it, Aura," Dr. Wells says, sounding excited by this achievement. "You're raising your body temperature. Keep going."

My eyes flicker open as the feeling intensifies, my insides swirling with heat. But there is more than just warmth in the pit of my stomach. There is fire and fury and a desperate desire to burn until Marine World catches alight.