It started as a tingling in her fingertips.

Clarke brushed the hair out of her eyes as she sat up in bed, examining her hands in the early morning light. A few moments of inspection revealed nothing noticeable, so she chalked it up to a sensation leftover from the mysterious ingredients in last night's moonshine. She was the first of the former prisoners to turn eighteen on the ground. Reaching adulthood without getting floated, according to Octavia, was cause for a celebration.

"Happy birthday, Clarke!"

Speak of the devil. Clarke straightened up as she emerged from her tent and came face-to-face with a grinning Octavia, who grabbed her hand. The tingling sensation traveled up her forearm.

"What's this?" Clarke asked as Octavia slipped something on her wrist.

"It's a flower bracelet. I made it for you, you know, since we haven't discovered an old diamond mine yet."

"Thank you. It's lovely," Clarke said as she brushed the velvety petals and tried to ignore her tingling skin. She looked up at Octavia and smiled. "Maybe we'll find those diamonds by the time you turn eighteen."

"A girl can dream," Octavia murmured as she wandered off.

Clarke grabbed an apple on her way to the dropship, fielding a few more birthday wishes along the way. Once on the lower level and out of direct sunlight, she glanced down at her hands and gasped. Her fingertips were glowing.

"Monty? Are you in there?" Clarke called breathlessly from outside his tent, her hands shoved inside her jacket pockets.

Monty peeked his head out from the opening and grinned widely. "It's the birthday girl!"

Clarke forced a smile. "Can I come in?"

Monty pulled the edge of the tent back. "What can I do for you?" he asked.

Clarke waited until the fabric dropped back into place. She pulled her hands out of her pockets and they both gasped. The glow had climbed from her fingertips to cover her palms.

Reaching out tentatively, Monty poked her left hand and recoiled.

"What?" Clarke demanded. "Did you feel something?"

"No," Monty said sheepishly. "I was just being careful."

Clarke released a frustrated breath and put her hands on her hips. "I need to know what's causing this. It must have been something I ate or drank. What did you put in the last batch of moonshine?"

"Nothing new. And I ate the same stuff you did yesterday," he said, trailing off as he examined his own hands. He held them up defensively. "And I'm fine. Maybe you just need to wait until it passes through your system."

"Let's hope you're right. I have too much to do to worry about this." Clarke turned to go. "And Monty, don't mention it to anyone, okay?"

By midday the tingling had reached her shoulders and was bordering on painful. Thankfully her clothes covered most of the faint glow beneath the surface of her skin. Clarke hid on the top level of the dropship under the pretense of organizing supplies. Really, she didn't want to frighten anybody.

Clarke willed herself not to be scared. Radiation was a very real danger on the ground, but so far they had not suffered any of its effects. And the Grounders weren't exactly bioluminescent.

She was ripping strips of fabric to make bandages when the tingling reached her breasts. Clarke braced her palms on the table in front of her, squeezing her eyes shut as the sensation consumed her. Her ability to keep this thing a secret was fading fast. She dropped into a chair and tried to catch her breath.

Eventually thirst overwhelmed her and she ventured outside. (If she was going to die of this tingling, or glowing, or whatever it was, she was not going to complicate matters by being dehydrated.) By this point the tingling had taken up residence in her stomach. Any farther and it was going to be difficult to walk.

"Happy birthday, Princess. Haven't seen you around this morning."

"Bellamy," Clarke said in greeting, tucking her hands behind her back as he approached the water tank. "I've been in the dropship." She was pleased with how even she managed to keep her voice, considering the tingling was about one centimeter away from her pelvis.

"Sorry I don't have a present for you," he teased, his eyes warm. "Octavia said she gave you a bracelet."

Clarke blinked at him as he looked down at her expectantly, but she kept her hands locked behind her.

"C'mon, let me see it," Bellamy demanded good-naturedly as he reached for her.

Several things happened at once when Bellamy's fingers closed around Clarke's wrist. A wave of heat rushed over her, washing away the painful tingling and replacing it with a pleasurable warmth. She inhaled sharply as she felt the glow that had been simmering under her skin intensify. Bellamy's eyes widened in shock just as Monty came running across the camp yelling her name.

As Monty pulled her out of Bellamy's grasp and into the dropship, Clarke glanced over her shoulder. Bellamy stood by the water tank, his lips parted in disbelief. His hand, suspended in midair where he had been holding her wrist, was glowing.

"I know what's going on with you." Monty said breathlessly as he turned around to look at her, the parachute falling back into place behind them and concealing Bellamy from view. "Clarke! Are you okay?"

Clarke was trembling almost violently as the tingling returned in full force. "It's getting worse, Monty," she managed through clenched teeth.

"Um, it's probably going to get worse before it gets better." He gulped. "I saw Lincoln when he came to see Octavia and I asked him, you know, about the glowing." He gestured at her hands, as if Clarke had forgotten.

"And?"

Monty took a deep breath. "Since you're eighteen now, it's time for you to find…a mate."

"A mate," Clarke repeated dumbly.

"Yeah. The tingling, or whatever, is going to keep getting worse until you find him. Lincoln said you'll know when he touches you."

Clarke stared at him, realization dawning. "Oh, no. No, no, no, no, no." She began pacing across the floor of the dropship, her glowing fingers gripping her hair.

"Clarke?" She stopped and looked at him. "That's not all. To get it to go away, you have to, um, you know." He glanced nervously at the floor. "Consummate it."

Clarke made a helpless sound somewhere between a groan and a whimper. "Monty," she whispered. "I know who it is."

You'll know when he touches you.

Night was beginning to fall as Clarke sat huddled on the floor of the dropship, cursing Earth and Bellamy in equal measure. The tingling engulfed her entire body, pricking her skin, although the glow had faded somewhat after her encounter with her apparent mate. She shuddered. Monty had been dispatched to Lincoln's cave for some answers – namely, how to avoid the whole consummation issue.

"Clarke? We need to talk to you." Octavia burst into the dropship, dragging Bellamy behind her. "My brother's hands are-." She stopped short when she saw Clarke curled up on the floor.

With considerable effort, Clarke looked up at Octavia's shocked face. She gritted her teeth against the pain before replying. "Mine too. Didn't Lincoln tell you?"

"No, I…" Octavia swallowed at looked at Bellamy, who was being unusually quiet.

When Clarke's gaze landed on him, Bellamy was staring back at her. She could see her confusion and fear mirrored in his face. When he took a step forward, the glow in his hands brightened.

Clarke gulped and glanced at her own hands, her fingertips lighting up in response to his proximity. When she looked up again, he was lowering himself to kneel in front of her.

"Clarke," he said gruffly, looking at her insistently. "What is this?"

Clarke made a small sound in the back of her throat but didn't answer. She wanted to move away from him, wanted to avoid having to face this thing connecting them. Instead she closed her eyes and let the nearness of him ease the tingling. She blinked them open when she heard him speak again.

"Look at your skin," Bellamy marveled. He passed his hand over hers, not touching, and they watched as a wave of bright light followed in its wake.

Octavia cleared her throat behind them. "What should we do?"

Without looking up from their hands, Clarke said, "Find Monty. He'll know what's going on." Octavia's footsteps receded as she hurried out of the dropship.

Clarke looked up at Bellamy. "It feels better like this. When you're close," she murmured.

"It was hurting you?" he asked, still bewildered.

"Yes." Clarke swallowed. "The only time it didn't hurt is when you…touched me. Earlier, when you were getting water."

"That's when it started," Bellamy said quietly, almost to himself. She saw the question in his eyes as he glanced from her face to their hands to her face again. His fingers reached tentatively for hers.

The feeling of being touched by him was almost too much for Clarke after so many hours without it. The tingling pain was replaced by instant, overwhelming relief. She let out a whimper as an intensely pleasurable sensation skated across her skin, bringing out a bright glow. Desperate not to lose it, she grasped on firmly to Bellamy's hand.

Bellamy was watching her curiously when she managed to meet his eyes again. "What does it feel like for you?" she asked breathlessly.

He stared at their joined hands. "Warm. Good." He flicked his eyes back up to her face. "Probably too good. This can't be normal."

"Earth is full of surprises." Her comment earned a smirk in return.

"What do we do now, Princess?"

"You can go," she said nervously. "I'll wait for Monty and we'll figure out what to do."

Bellamy tilted his head. "I don't know, Clarke. You didn't look so great when I came in here."

"Well, I don't want to hold your hand for the rest of my life," she quipped. "Let's just try to separate, okay?"

Bellamy glared at her before dropping her hand and edging away. He watched as the glow faded slightly from her skin and, upon further inspection, his as well.

Clarke felt fine until Bellamy got about halfway across the dropship. Then the tingling returned, stronger this time. She scrunched up her face in pain and doubled over. Bellamy was back at her side in an instant, nearly lunging for her hand.

"Clarke? Clarke, look at me." He grasped her fingers harder but she was still trembling.

"It's not enough," she breathed. "I need more."

Bellamy hesitated for a moment before scooping her up and setting her in his lap. He smoothed one hand down her the back of her jacket and then up under her shirt, against her bare skin. With the other he brushed her hair off her shoulder to allow him to rest his hand against her neck.

Clarke sighed as warm relief washed over her once more. "I'm in so much trouble."

Monty and Octavia found them curled up together in the dropship. Clarke was trailing her fingers down Bellamy's arm serenely as he rubbed soothing circles on her back. Both of them were glowing brightly.

Clarke looked up. "Monty, you're back." She glanced at Octavia with trepidation. "Did he…tell you what's going on?"

Octavia chuckled. "Oh, yeah. You and my brother are in so much trouble."

Bellamy snapped his head up. "Why me?"

Clarke pressed her lips together. "Lincoln said that since I'm eighteen today, I need to find, um, a mate." She mumbled the last two words so softly that Bellamy couldn't hear her.

"Find what?"

"A mate!" Octavia nearly shouted. "And you're it!"

Monty, ever the scientist, jumped in. "Clarke's the first one of us to turn eighteen, so we didn't know this until now, but whatever's going on with her is part of her becoming an adult and finding a partner. The tingling stops when you touch her. So you're it. You're her mate." He paused for a breath and switched his glance to Clarke. "Did you tell him about the other part yet?"

"No," she hissed. "I was kind of hoping you would find a solution."

"Lincoln said that if you can make it until morning, you'll probably be fine tomorrow."

"What do you mean, make it until morning?" Bellamy asked.

Monty shuffled his feet and looked at the floor. Clarke sighed – it was her problem, after all. Turning to face Bellamy, she said, "Apparently Earth wants me to have sex with you. You know, to complete the mating ritual."

Bellamy froze and tightened his arms around her. His expression was unreadable.

Clarke sighed again. "Don't worry. It's not happening. I can deal with this" – she gestured at her body – "until morning. Octavia will stay with me, right?" She looked up at the younger girl.

"Yeah, sure," Octavia replied, smirking. "I wanted a nightlight anyway."

Her resolve back in place, Clarke moved to extricate herself from Bellamy's arms, steeling herself for the pain that would surely follow. She tossed one last look over her shoulder and exited the dropship behind Octavia. Bellamy and Monty stared at each other before following suit.

"Clarke," Octavia said as they walked side-by-side, Clarke's hands stuffed inside her pockets. "Would it be so bad if Bellamy were your mate? I mean, we're surrounded by juvenile delinquents. You could do worse."

"Octavia, he's not my mate. That's ridiculous. This is just another weird Earth thing that seems terrible while it's happening but passes quickly. Like the acid fog."

"Are you comparing my brother to acid fog?"

"No, I just-." Clarke grunted as a particular intense wave of pain skipped over her skin. "I just need to get this under control until tomorrow."

Octavia furrowed her brow. "Why don't you let Bellamy help you? You obviously felt better when he was touching you."

They reached Clarke's tent and ducked inside. Clarke collapsed on her makeshift bed. "Because." She let out a breath. "I need more from him every time we touch to make the tingling go away. It's leading somewhere. Somewhere I don't want to go."

Octavia flopped down next to her. "Haven't you ever thought about it? You guys spend so much time together as it is."

Clarke let the question hang between them, examining her glowing fingertips. "Yes. But if it happens, we have to choose it. On Earth, we're finally free to make our own choices. I don't want this, whatever it is, to take that away."

"Maybe this is confirmation," Octavia murmured, her eyes drooping closed.

Clarke turned her head. "What do you mean?"

Octavia didn't answer and Clarke assumed she had fallen asleep. She was about to turn away when she heard Octavia whisper.

"That you already made the right choice."

Clarke stared at the ceiling of her tent as Octavia slept. Every time she nodded off a fresh wave of pain woke her up again. She had never been so ready to get her birthday over with.

Her jacket now off, Clarke watched the faint light pulse beneath the surface of her forearm. Almost imperceptibly, it began to brighten. He was close.

Suddenly a glowing handprint appeared against the tent flap, and Bellamy poked his head inside.

"Bellamy," Clarke hissed. "Get out of here."

"Clarke," he whispered harshly, pain clearly etched on his features, "I need you." His glowing fingertips reached for her.

"No!" Clarke said, louder this time, before glancing at Octavia to see if she had awoken. "Just – don't touch me, okay? I'm coming outside."

Bellamy backed up and allowed her to exit. She was careful not to brush against him.

"We need to go somewhere," Clarke insisted. "Anyone could see us."

Bellamy led Clarke to his tent and lifted the flap for her. They were barely inside before she whirled around to face him.

"What do you think you're doing? Didn't you hear me earlier? I can't be near…" Clarke trailed off and inhaled deeply at the new sensation of being in his tent. It smelled like him. She felt like she was being slowly wrapped in a warm cocoon, the tingling fading fast.

Their proximity seemed to be having the same effect on Bellamy. His chest was heaving and the exposed skin on his arms was brightly illuminated.

He licked his lips. "It started hurting, when you left. I wanted to…touch you again."

"I know it feels good," she said, trying to steel herself against the wanting look in his eyes. "But we can't. It only ends one way."

"No, Princess. It doesn't have to. We just need some ground rules."

Clarke arched an eyebrow.

Bellamy rubbed a hand against his neck, leaving behind streaks of light as his fingers trailed away. Clarke was momentarily mesmerized.

"We can touch each other to ease the pain," he continued, "as long as we don't let it get too far." He smirked. "I don't want you taking advantage of me just because you think you've found your mate."

"Look, I don't think you're-."

Bellamy held up a glowing palm, cutting her off. "Okay. Rule Number 1: No kissing. Rule Number 2: Pants stay on."

Clarke's eyes widened. "Those are the rules?"

"That's all I came up with before I went to get you, yeah."

Clarke rolled her eyes. "I like my plan better."

"Really?" Bellamy smirked. "I think I can convince you."

Before she could react, he grabbed her by the hips and pushed his hands underneath the hem of her shirt. Clarke shoved him away almost immediately but not before a wickedly pleasurable heat wrapped around her waist.

Clarke glared at him and struggled to keep her bright hands from trembling. "Rule Number 3: You never mention this to anyone. Got it?"

Bellamy grinned.

"I think you should take off your shirt."

Clarke scoffed. They were side-by-side on his bed, shoulders touching, hands clasped. She felt strangely formal, like they were posing for a photo lying down.

The limited contact clearly wasn't enough for either of them, but Clarke didn't want to take any chances.

"Here. I'll go first." Before she could stop him, Bellamy sat up and pulled his shirt over his head, tossing it away. He picked up her hand again and placed it on his bare chest.

Clarke swallowed her protests at the new sensation. The warmth flowing up her arm felt unnaturally good. She moved her fingers experimentally, unexpectedly delighted at the designs they left behind on his glowing skin.

Clarke looked up at him to find that he was already staring at her. He gave her a lopsided smile.

"C'mon, Princess. Don't you want to feel this?"

She lay back and reached her arms over her head, trusting him as he glided her shirt up and off.

Bellamy ran his hands down her arms as he returned them to her sides, and Clarke nearly hummed with pleasure. He spread one hand across her bare stomach, rolling pressure through from his wrist to his fingertips until he was satisfied with the handprint he left behind.

Then he leaned down and kissed it, letting the tip of his tongue touch her soft skin.

Desire shot through Clarke like lightening, swift and hot. She let out a strangled sound and sucked in her stomach. "Bellamy! I thought there was no kissing!"

Bellamy shrugged. "I wanted to try it. It's not like I kissed you on the mouth."

Clarke narrowed her eyes at him. "You're going to pay for that."

Without preamble she pushed him on his back and licked a slow circle around his navel, feeling triumphant when he shuddered underneath her.

Bellamy was quick to retaliate. Keeping his eyes locked on hers, he grabbed her hand and sucked her first two fingers into his mouth.

"Oh," Clarke stuttered. Things were getting out of hand – they were definitely bending the rules if not breaking them altogether – but the rational part of her brain had surrendered to the soft sucking sounds Bellamy was making as he laved her fingers with his tongue.

When he let them slide out of his mouth, they were glowing bright red. Clarke trailed them down his sternum as fleeting bits of sensation zipped across her skin like aftershocks.

They stared at each other, equally breathless. Clarke was the first to move, leaning down and pressing her lips to his.

Kissing Bellamy, even innocently, was everything she feared – unprecedented pleasure and a sinking knowledge that here, in her embrace, was her perfect match.

Bellamy's mouth had been all over her torso. Clarke whined when he was away from her lips for too long. She craved the feeling of his tongue sliding against hers, causing fiery sensations to lick at her skin. Bellamy seemed happy to indulge her.

Their skin glowed brighter than ever, but Clarke was too far gone to care about anyone seeing the illuminated tent. She moved her hand down his chest to the waistband of his pants.

Bellamy covered her hand with his own and lifted his mouth from hers. "Clarke, hold on." He waited until she opened her eyes. "Pants stay on, remember?"

"Is that what you want?" she breathed.

"No." Desire glinted in his eyes, competing with hesitance. "I know you don't believe in the whole 'mate' thing" – she turned her head away and he gently touched her chin until she faced him again – "but something tells me that this can't be undone."

"No," Clarke whispered. "Whatever it is, I think it's permanent."

Bellamy swallowed and cupped her cheek. Sparks danced across their skin. "I'm in."

"Me too."

With nothing separating them anymore, the glow from their bodies reached the treetops and disappeared into starlight.

"Rise and shine, lovers!" Octavia burst into Bellamy's tent with a grin on her face. "You two have some explaining to do. Your tent looked like it was on fire last night!"

Clarke buried her face in Bellamy's shoulder as his arm tightened around her.

Bellamy groaned. "O, get out."

"Don't hate me because I was right! And just wait until my eighteenth birthday." She skipped back outside.

Clarke rubbed her eyes. "Right about what?"

Bellamy pressed his lips together and sighed. "She knew that we would end up together. Even before this whole…thing happened."

"Bellamy, this wasn't a fluke. I wanted you. It was a choice."

"Well, I hope you made the right one, Princess. Now you're stuck with me."

Clarke rolled her eyes. "At least I'm not glowing anymore."

Bellamy brushed a strand of hair behind her ear. "Yes, you are."

Later that day, Clarke came up behind Bellamy and slipped her hand into his. He looked down at her warmly, the corners of his eyes crinkling.

"Hey, Princess. I thought you didn't want to hold my hand for the rest of your life."

Clarke grinned up at him. "Well, this planet seems to be one big deathtrap. So I won't have to tolerate you for too long."

Bellamy laughed and leaned down to kiss her. The sun filtered down through the trees, warming their skin. Making them glow.