Cold coffee tasted like disappointment. Callie swirled the lukewarm liquid in her paper cup, staring blankly at the beige breakroom wall. Every muscle in her back ached from a five-hour spinal reconstruction she had wrapped up at dawn. Twenty-four hours on call had drained every ounce of her patience, leaving her raw and irritable. "Torres!" a voice barked from down the corridor. Bleeping pagers rattled against her hip, a frantic rhythm demanding her immediate presence in the emergency room. She tossed the half-empty cup into the trash and sprinted down the fluorescent-lit hallway, her sneakers squeaking loudly against the polished linoleum. Rain poured outside, slicking the ambulance bay concrete and reflecting the flashing red lights of three incoming rigs. "What do we have?" Callie grabbed a yellow isolation gown, snapping her latex gloves over her wrists with a sharp, echoing pop. "Eight-year-old male, collision with a delivery truck while riding his bike," the paramedic shouted over the siren's wail as they wheeled the gurney inside. Screams of pain tore from the little boy's throat, a raw, ragged sound that made Callie's chest tighten. Sliding alongside the gurney, Callie immediately assessed the damage, her eyes locking onto the twisted right leg. "I need orthopedic surgery, now!" she yelled, guiding the team into Trauma Room One. "The tibia is completely sheared, and I'm losing the distal pulse." Moving fast, she began cutting away the boy's soaked jeans, revealing a jagged bone protruding through the blood-slicked skin. "Get me a traction splint and prep OR three," Callie ordered, her fingers searching for a pulse in the boy's foot. "Step aside, Ortho." Sweet, melodic, and entirely too cheerful for a chaotic trauma bay, the voice cut through the panic. Callie blinked, looking up to find a woman standing at the foot of the bed. Bright blue eyes stared back at her beneath a scrub cap covered in cartoon giraffes. Dimples dented the stranger's cheeks despite the blood and tension filling the room. "Excuse me?" Callie snapped, her grip tightening on the traction splint. "This is an open tibia fracture with vascular compromise. I'm taking him up right now." "And I'm the new Chief of Pediatric Surgery," the blonde replied, stepping forward with an easy, unyielding authority. "Which means this little guy is my patient first." Silence fell over the attending residents, everyone suddenly holding their breath. "He is bleeding out from what looks like a femoral laceration," Callie hissed, her jaw locking as she stepped closer, towering over the blonde. "He doesn't have time for a pediatrician to blow bubbles and hand out stickers." "I don't blow bubbles, Dr. Torres," Arizona said, her voice dropping its sweet tone, replaced by something dangerously sharp. "I save children. And right now, his rigid abdomen tells me he has a ruptured spleen. He will bleed to death on your ortho table before you even skin prep his leg." "No way," Callie growled. "Yes way," Arizona countered, stepping so close Callie could smell her vanilla perfume beneath the heavy scent of antiseptic. Fighting for dominance, they stared each other down over the crying child, the air between them practically crackling with static. "Listen to me, giraffe cap," Callie said, her voice dropping to a dangerous whisper. "If I don't restore blood flow to this leg in the next twenty minutes, he loses it." "Let go of the gurney, bone doc," Arizona replied, her blue eyes flashing with a sudden, fierce heat. Before Callie could retort, a commanding figure stepped into the trauma bay, his face set in a stern grimace. "Stop arguing like children and save the actual child," Chief Webber boomed, crossing his arms. He looked at both women, his eyes narrowing. "Both of you are going up," the Chief ordered. "Dual-occupancy OR. Torres, you fix the leg while Robbins patches the spleen. Move!" "Fine," Callie spat, grabbing the head of the gurney. "Perfect," Arizona smiled, though the expression didn't reach her eyes as she took the side. --- Minutes later, the sterile air of OR Three washed over them. Inside the operating room, the atmosphere was thick with tension as the two surgeons scrubbed in. "Play some music," Arizona ordered the circulating nurse. "No music," Callie countered immediately. "I need concentration." "Oh, come on," Arizona scoffed, stepping into the room with her hands wet. "Some upbeat pop never hurt anyone." Upbeat pop music began to blast through the speakers, a bouncy, bubblegum track that made Callie want to hammer her own head against the wall. "Are you seriously going to make me listen to this while I reconstruct a tibia?" Callie groaned, holding her hands up as the nurse gloved her. "Happy surgeons make fewer mistakes, Dr. Torres," Arizona said, her eyes smiling above her mask as she took her place at the boy's abdomen. Focusing on the task, Callie forced herself to ignore the cheerful rhythm of the music. "Scalpel," Arizona ordered, her hands moving with lightning speed and incredible precision. "Clamp," Callie countered, her own hands steady as she began the delicate process of aligning the bone. Working in tandem, they moved like two opposing forces forced into a delicate, dangerous partnership. "Your hands are remarkably steady for an ortho surgeon," Arizona noted, her tone shifting from playful to genuinely impressed. "Thanks, I think," Callie murmured, adjusting the external fixator. "I thought pediatric surgeons just used band-aids and wishful thinking." "I meant they are gentle," Arizona clarified, looking up from the abdomen, her blue eyes softening. "Most ortho surgeons treat bones like construction sites. You treat them like glass." Sarcasm dissolved in the quiet warmth of the compliment, leaving Callie briefly speechless. Bone alignment was completed just as Arizona successfully repaired the splenic tear. "Nicely done, Dr. Robbins," Callie admitted, stepping back to let the residents close. "You too, Dr. Torres," Arizona replied, pulling down her mask to reveal those deep, infuriatingly beautiful dimples. "See? We make a great team." --- Hours flew by in a blur of post-op checks and chart dictations. Exiting the hospital doors late that evening, Callie felt like a walking corpse. She needed a hot shower, a giant glass of red wine, and twelve hours of uninterrupted sleep. Passing through the wet parking lot, she climbed into her car, the heavy Seattle rain drumming a relentless beat against the windshield. Stepping out of her car outside her apartment building twenty minutes later, she pulled her jacket tightly around her shoulders. Rain was still falling, turning the pavement into a slick mirror. Exhaustion dragged at her limbs as she made her way up the three flights of stairs to her unit. Climbing the stairs, she fumbled with her keys, her mind already drifting to the comfort of her bed. Unlocking her front door, she pushed it open, expecting the dark, quiet sanctuary she had left that morning. Warm light spilled from the kitchen, casting long shadows across the hardwood floor. Callie stopped dead in her tracks. Soft jazz music was playing quietly from a speaker on the counter. "Hello?" Callie called out, her hand instinctively gripping her heavy purse like a weapon. Rounding the corner into the living room, her breath caught in her throat. Arizona Robbins was standing there, holding a half-empty glass of white wine. She wore an oversized gray sweatshirt and thick woolen socks, her blonde hair pulled back in a loose, messy bun. "What are you doing in my apartment?" Callie demanded, her heart hammering against her ribs. "I could ask you the same thing," Arizona said, tilting her head as a slow, knowing smile spread across her face. Callie stared at her, her brain stalling as she processed the sight. "This is my place," Callie said, her voice rising. "I live here." "I know," Arizona whispered, taking a slow sip of her wine, her blue eyes gleaming with mischief. "Why are you here, Robbins?" Callie asked, her knuckles turning white. "Because," Arizona said, leaning against the kitchen counter. "Your landlord sublet the spare room to me this morning. Looks like we're roommates, Torres."

Chapter 1: Ground Rules