A/N: This would have been my second idea, but I have this one mapped out already and am too exited to write it to let it fall to the wayside.

Summary: A dark secret from Castle's past resurfaces just as a high-profile case puts him at odds with Beckett. (Set late season 3, post-Countdown but pre- To Love and Die in LA.)

"Ethan Wilson, you're under arrest for the murder of Thomas Hughes."

Beckett reads off his rights as she tightens the handcuffs around his wrists then pulls him off the wall of the closed down shop just outside of a park on the other end of town. She leads the suspect out with Castle close behind her, handing him off to Esposito, who's just coming out from a corner with his rifle hanging from a sling on his chest. "Where're your friends, Wilson?" Espo asks him as he takes his arm and pulling him outside into the sun.

Castle follows her outside as she re-holsters her weapon. "We have an APB out on them. We'll get them."

A group of three guys in their mid-twenties had broken into a cosmetics factory on a false tip that they had animals that they were using as test-subjects, intending to free them. Wilson wasn't their ring leader but was the one who ended up pulling the trigger on the elderly security guard. Having met in college in a group of peaceful animal rights activists, the group seemed mostly benign and was just a social club until a few of them got overheated when a jaded ex of one of the boys had lied and told them that there were animals being abused in the facility they broke into, hoping they would get sent to jail.

She had confessed with large, crocodile tears after Beckett had made her crack after just a few minutes in the box. They had tracked Wilson to this closed down shop just off the park, hoping that his two partners were inside. According to their social media, they were prone to violence and had a very public disdain for law enforcement.

"All this because of a jaded ex." Castle says as Esposito pushes Wilson down into the patrol car.

"Well, you should know all about that, Castle." Beckett cracks with a grin as she taps his arm with her elbow and moves through the park to question the owner of the building if he'd seen any sign of Wilson's partners. Him, along with the rest of the pedestrians are being kept a good distance.

Castle fights a smile as he watches her go if nothing else because of her grin as she jokes with him. He shakes his head and goes to join Esposito, who has the door open to the patrol car with Ryan next to him, questioning Wilson. "I'm telling you, I don't know!"

"Really? You have no clue where the other two little rascals are?" Esposito asks with his hand resting on the stock of his rifle.

"We decided to split up whenever we heard that you were looking for us. They-" He cuts himself off and looks forward sternly.

"What's the matter, Wilson? Cat got your tongue?" Castle asks with a smirk.

Wilson just sends him a death glare and doesn't answer. "Alright, get him out of here." Esposito says and flings the door closed. "We'll get it out of him at the precinct."

Ryan follows Esposito out along the street as Castle looks toward the park. "Hey, Castle, where's Beckett?" Ryan asks as he turns around, walking backward.

He steps out into the sun and finds her walking with her eyes turned down to her phone, just stepping onto the sidewalk that makes for the border of the park. He lets out a grin and takes a step forward. He's only able to take two steps until he sees a dark sedan speed up behind her, the two doors on the driver side flying open with two men clad in black masks jumping out. "Beckett!"

The three stop as they watch one of the attackers thwack Beckett in the back of the head with a gun the other grabbing her shoulders and picking her up. Ryan and Esposito are quickly running forward, Castle behind them as the rear driver-side door closes with Beckett still inside. He can hear Ryan calling on his radio for back-up, but the car is already starting to take off.

Castle's heart and mind harden as the air in his lungs turns cold. He can't let this happen. "Give me the rifle, Esposito." He demands.

"No, Castle, we have plates. We-"

"Give me the rifle!" He yells, his eyes following the dark sedan as it speeds down the street on the other side of the park.

"Castle!"

He can't take this anymore. He has to do something. Grunting out through his clenched teeth, he grasps Espo's sling in one hand and the grip of the assault rifle in the other, unclipping it from the sling and then takes off running as fast as he can, holding the rifle by the grip and the magazine well. "Castle!" Ryan calls.

"Castle!" Espo calls after him again.

But he's not listening as he pulls the charging handle back to see a round in the chamber, his legs sprinting quickly through the park. "What the hell is he doing?" Ryan asks Esposito.

But Esposito watches intently. There's something about Castle's movements that seem too familiar to ignore. Castle runs swiftly through the park as the dark sedan that just kidnapped Beckett nears an intersection.

Castle's eyes follow the sedan, his eyes tracing the outline of the driver as he makes his way down the street. He takes a few more long strides until he skids himself to a stop in the grass behind a park bench. He lifts the rifle up and presses the stock into his shoulder tightly, his hand gripping the top of the magazine well as he looks through the red-dot sight.

His heart is pounding, but he holds his breath as the sedan slows down to make the corner, still going a good twenty to twenty-five miles an hour. His eyes narrow as he takes aim and squeezes the trigger. An instant later, he looks down the sights to see a light cloud of red mist in the air puffing out of the windshield. The car goes hectic and speeds up until it rams into a streetlight, the horn starting to blare as it stops.

"Oh my god." Ryan says in shock and amazement as he watches Castle make the shot.

Esposito's brow furrows as he watches Castle's movements. There's only a handful of people who could have made that shot.

But Castle stays knelt down behind the bench, the barrel of the rifle resting against on the top for stability, waiting for the other to appear. And as the rear driver-side door opens, looking down the sight at the other suspect as he has his back turned toward the front of the car and starts to stumble out, his gun raised in Castle's direction.

The target fires a shot at Castle, making Ryan and Espo flinch at the sound. But as Espo's eyes train themselves on Castle, he doesn't move as he fires another shot, putting another quick double-tap to the center of the target's chest. The other suspect falls out of the car and onto the pavement. Castle lets himself breathe as he stands up and takes off running toward the sedan with its horn still blowing.

He can feel his heart again after icing himself over as he quickly moves around to the passenger side and opens the door. Beckett falls into his arms as the door opens. "Kate?" He asks softly.

He pulls her out and picks her up in his arms, her head falling away and her body limp. He quickly and carefully kneels down, lowering her body down to the pavement, his hand under her head as he feels her limp, unconscious head turn about in his hand.

His heart palpitates in his chest at the thought of losing her, the one woman he's felt he's ever really been in love with. His eyes close and he presses his fingers softly into her throat with his thumb petting her jaw line, feeling the bump of her pulse against them and he feels a deep, relaxing sense of relief wash over him just as Espo and Ryan come running up from around the other side of the car. "She needs a medic, now."

"They're on their way." Ryan starts, "Castle, how did you-"

Esposito grabs Ryan's vest and shoots him a look not to ask and kneels down next to Castle as the sirens start to wail in the distance.

Once at the hospital, vests off, Castle paces impatiently in the waiting area.

If it was any other situation, he probably wouldn't have taken action. But this wouldn't be the first time he's let this quasi-facade fall in the name of making sure she saw the next day. He's done it before. Hustling her at the shooting range out of a file he wanted, shooting that gun out of Dunn's hand, punching out Lockwood.

But this time, he might have let too much of that facade fall. He could tell that Esposito was holding his words back, and telling Ryan to do the same. What he feared most, he felt was about to unfold thanks to the necessity of his actions needing to be taken. But from what the doctor told him, Beckett should be fine. Nothing serious but a blow to the head. They don't think it's a concussion but they have to check to be sure. At the most, she'll have a splitting headache for a couple days.

Which will be great for her moods.

"Hey, Castle."

He closes his eyes and lets out a silent sigh of frustration, turning to Espo with a smirk. "The doctor's said she'll be fine. Just a bump on the head is all."

Espo nods and raises his brow, tapping him on the arm. "That's good." He says, nodding slowly. "That's real good bro."

Castle smiles and looks down to the floor. Something about Espo's eyes seem... too knowing.

"Hey, uh..." Esposito says, pushing his hands into his pockets and shifting his weight. "That was a hell of a shot you pulled off."

Castle shrugs his shoulders innocently and starts to pace away from him. "Just got lucky, I guess."

"So uh..." Esposito says, the sound of his voice telling him that he's starting to follow. "Where'd you train?"

Castle whips back around to look at him with a knot in his brow and his stomach. "W-what are you talking about?" He asks on a humorless chuckle.

"Come on, Castle." He replies with a smile. "I saw the way you moved out there. I know military training when I see it."

"I'm a writer, Esposito." Castle tries to throw off.

But Espo doesn't seem to buy it. This is what he was afraid of. "Castle, there's only a handful of people that could have made that shot and they're all in the military. So, where'd you train?" Esposito says with an open, all too knowing smile.

"I... I wasn't in the military." He says with a tight jaw, pacing passed him.

"Castle, drop the act, will you?"

"It wasn't... military!" Castle urges him, turning back to him with his hand out in front of him. Castle meets his eyes and feels his gut twist. "It was..." He closes his eyes and shakes his head. Maybe he can keep it between just them. "...Private security."

Esposito's brow lowers and his eyes harden as Castle paces passed him again, restless and unable to stand still. "You were a turncoat?!"

"Privately hired security liaison!" Castle spits, turning back to him with clenched teeth. Esposito's expression doesn't shift from anger as he waits for Castle to explain. Castle pushes out a breath through his nose and rolls his eyes. "I was a high school graduate with no real-world ambitions and no real family I would be leaving behind. Someone who had graduated a few years earlier offered me a job and the money was too good to pass up. It would be a few months of training and two years of service, then the contract would end and I could go home. So... that's what I did. Did my job and used the money to get In a Hail of Bullets published."

Esposito just stares him down with a furrow in his brow.

"It's ancient history that I've put behind me." He finishes and turns away from him.

"Well, does Beckett know?"

Castle looks over his shoulder to him, "Of course, Beckett doesn't know. She'd be the last person I would tell this to."

"Castle, you have to tell her about this." Esposito says.

"That's not gonna happen, Espo." He says and paces away from him.

"Well, if you don't then I will."

Castle reacts quickly, grabbing Esposito by the collar and shoving him against the wall, hissing at him through clenched teeth. "If you utter one word about this to Beckett, I'll do things to you they didn't teach you in boot camp!"

After a tense second of Castle holding Esposito against the wall in a tight grasp, he lets him go and Esposito shakes him off.

"I did things I am not proud of, Javi. And unlike you, I don't have the luxury of telling myself I was just doing my patriotic duty. And if anyone can understand why I put it behind me and how hard it was, it should be you." Espo looks away and seems to un-tense, seeming to understand where Castle is coming from. "Beckett can't know about this, okay?"

"Well, what about the report? Me and Ryan both saw you make that shot. She's gonna have questions, Castle."

Castle pushes out a hard breath and looks down the hall. "As far as the paperwork's concerned... you made that shot. Tell that to Ryan next time you see him, and..." Castle says, taking a step closer to him, "Javi, please... keep this between us."

Esposito fights off the deep crease in his brow but nods anyway. "Beckett will never know."