If you ever asked Jay Halstead who was his best friend and why, he would have told you, Greg Gerwitz aka Mouse. Why? There was a simple explanation for that. The guy had saved Jay's life over 10 times not only in Afghanistan, in Las Vegas and Chicago as well, preventing him from following the wrong path in his life. The detective could narrate their stories for hours, especially to his partner Hailey, who was curious to find out what so special about this man.

"Last time…" Before Jay finishes his phrase, the girl motioned to him with the finger that it was time for him to stop talking. "Sorry if I made you feel bored…"

"No, it's not that, the pizza will get cold." Grabbing a slice of pizza from its package, she gently shoved it into Jay's mouth to prove her point.

"You are right. Maybe we should watch that movie and eat."

After cracking a small smile, Hailey pressed the play button on the remote, taking a deep breath for what was about to follow up. Vivarium. A horror movie, not so scary but terrifying enough to make the woman scream. Even though being a cop required strength, courage, and braveness, credentials that Hailey disposed of, she acted like a small child every time she watched a horror movie. Therefore, today's bet. 100 dollars that Hailey wouldn't run of the door screaming.

At first everything seemed normal in the movie. A happy couple looking for a house, they meet with a strange man and everything from there goes downhill. As the minutes went by, Jay took several peeks at his girl. She was hugging a pillow and her gaze was stuck on TV. He didn't pay much attention to the movie though at first, but after the mystery started evolving, it was like he became addicted.

Suddenly a small but audible noise was echoed in the room, startling them both.

"What was that?" Hailey whispered clearly worried not paying attention to the movie anymore.

"That would be the door, but who could it be at 2 am in the morning?"

"Don't open it… Or at least take you a gun with you."

"Baby, you don't have to be afraid." He replied regretting his first word. Even though he loved Hailey, he hadn't admitted his feelings yet. Feeling uncomfortable enough, he lifted his body up and walked all the way to the door, still holding his beer.

For a moment silence dominated the room with a still Jay Halstead standing on the door, beer spilling the floor in small drops. The man couldn't move, talk, or react making his partner anxious. Approaching him slowly, she noticed a young man around his 35s in a military uniform standing there smiling. Jay was unmoving like a statue.

"Who is that?" She asking shaking his right shoulder a bit.

"Grez Gerwitz ma'am. Or Mouse. I know, it's a silly nickname but I am the first to admit it." He stretched his hand for a handshake which was received back in no time. "Are you good buddy?" He turned his gaze to his best friend.

No words needed, just a tight hug between the two men. Even though Jay stretched his injured shoulder causing pain to himself, he didn't care. Mouse was there, alive and nothing else mattered. Only when they broke the hug, he realized that maybe he would need his old sling again for a while. A groan escaped his hips despite the fact that he tried to avoid it.

"You okay man?"

"Yeah, yeah… It's an old injury… Don't worry about it. Come on, give me your luggage, you must be tired." Grabbing the bag Mouse was holding, he struggled to move, hearing the laughs of the two people behind him. "Oh yeah, go ahead. Laugh at me."

Assisting him with the bag, the three people sat on the couch while the movie was still on. For Hailey, this was a huge advantage because she wouldn't lose any money.

"Sooo… what are you watching?" Mouse asked looking at a weird town on TV where each house resembled to the other.

"Just a horror movie." Jay answered slowly massaging his shoulder.

Taking a peek at his surroundings, Mouse smiled. Even in a new house, Jay's old fashioned style was everywhere from the walls to the rugs on the wooden floor. The window behind the TV revealed a peaceful sleepy Chicago, a city that was deeply missed by the young soldier. "You know; I hadn't seen a clear sky in years." He exclaimed turning to the two people next to him.

"How come you're back?"

"You know how war is... One day you are completely healthy and the next you find yourself in a hospital bed for a while."

Fear was written all over Jay's face as soon as he heard that statement. "Why didn't you call me?"

"Come on man, Chicago needed you, there was no way that I would distract you from your job." As a soldier, you learn how to cope with downside events on your own, clearly cause there isn't any help in the battlefield sometimes. "Besides that was eight weeks ago."

A gasp left Hailey's mouth as the date brought back painful memories for her too. "Well even if you would have called him, he wouldn't be able to join you, he was preoccupied dying on the operating room at Med." She joked trying to expel every single minute of that day out of her mind.

Mouse looked at Jay full of a query.

"Nothing serious, I was kidnapped and got shot. Don't bother, I am fine."

"Okay… I guess you had a good company." He gestured to Hailey, winking.

Immediately she got up and stepped a couple of inches away from her partner. "It's not what you're thinking... Mouse. We are just friends… I am gonna get you a beer."

"I like her… Lucky dude." Mouse burst into laughter punching his friend gently on the back.

After hearing some stories, Jay had already told her and some new ones, very uproarious about her partner, the woman couldn't control herself and laughed hard receiving death glares from the man next to her. The three of them continued discussing till they all fell asleep on the couch.

The first blinking of the morning came from Hailey whose ears were busted with a loud noise of screams and cries. At first, she thought, it was Jay but her gaze turned to the other man who was probably caught in a nightmare.

"Mouse, wake up. It's a dream, only a dream."

His blue eyes slashed open wandering around the room until they met her eyes.

"It's just a dream…" she whispered again assisting him on his breathing. "Are you okay?"

He nodded swiping the sweat off his forehead. "I'm sorry."

"You don't need to… Do you want some coffee?"

"Sure ma'am."

"Hailey alright? Just Hailey."

Smiling she marched to the kitchen being followed by the young man. Hailey grabbed two large cups from the cupboard and turned on the coffee machine.

"I am sorry for before." Mouse exclaimed staring at the blank space in front of him.

"Don't worry, I am used to it…"

"Right." He smiled like a small child. "So, you and Jay…"

"No, we are not a couple if that's what you mean."

"But you love him."

"I do…"

"You should tell him. You know he's never gonna make the first move. Like ever."

Looking outside of the half-opened window, she stared at the sun which had started to rise. It must have been very early in the morning from what she figured. No sound was heard on the background, only the noise of the engine of some cars in the neighborhood.

"Jay…um… I don't remember him being such a deep sleeper."

"It's cause of the meds. He's out like a leaf every night."

"At least he gets to sleep."

Hailey filled the cup with hot coffee and handed it to him before she served herself.

"I can't wait to explore Chicago. It's been four years since I left." He took a sip of his coffee.

"What are you going to do from now on?"

"I don't know… Probably get a job and a house… The reintegration to the real world is always hard. When we came back the first time, it was two of us you know… We had each other for support but it's different now. Jay's life is stable and I don't want to be a dead weight to him."

Hailey shook her head.

"Believe me, Jay's life is nothing else but firm at the moment. Ever since he was released from the hospital, the constant pressure from Will and well the whole Intelligence has driven him nuts."

"I am glad you are here for him."

A sudden groan left them both stranded, making them turn their heads to the source of the sound. A completely awake Jay Halstead sitting up from the couch, stretching his body.

"Good morning sleeping beauty." Mouse joked.

"What day is it? What year?"

"Do you want some coffee?" His partner asked.

Jay nodded approaching the kitchen, rubbing the back of his neck, taking a seat next to his best friend, grabbing the coffee in front of him. "Are you coming to the district Mouse?"

"I would love too."

"Alright, folks, let's get ready and go to work before Voight gets angry." He announced drinking in a hurry.

The district was rather than quiet. Patrol offices men and women inserted and exited the building at a quick pace seeming preoccupied about something. Not only them, Trudy Platt as well. She was on the phone talking when the three people walked in trying to take in what had happened.

"Sarge what's going on here?" Hailey inquired while being accidentally pushed by Garcia who was running outside.

"They just murdered Jonas…" She replied not even paying attention to the third man of the company.

Alex Jonas, a new policeman from the academy with less than two months in the job. Jay and Hailey had met with him once or twice, in reality, no one knew him that well, however, the boy was one of their own so they had to take action immediately.

"Why nobody called us?" Jay wondered.

"It happened five minutes ago… I guess Voight was just about to inform you."

Without any other words, the three of them climbed the stairs leading to Intelligence, being met with three pairs of eyes. Rojas, Atwater, and Ruzek. Burgess wasn't there yet.

"Mouse." Adam and Kevin stood there surprised.

"Hey, guys."

"Hello Mouse, welcome back." The groggy voice of Hank Voight was echoed in all over the room pulling the attention to him. "I would love to greet you properly but I am sure you found out what happened. Alexander Jonas. 25. Police officer. He was murdered in lower Wacker and Columbus a couple of minutes ago. We wait for the witnesses' testimony so we can move on. Everybody suit up.. We'll go inspect the scene.

"Is there anything I could do Sarge?" Mouse asked ready to go back in action even off the books.

Considering that he was a reliable and expert hacker, he would be a useful addition to the team at the present time. "Alright, come with us… You know what you have to do."

Receiving back, a secure nod, the older man descended the stairs to the garage being followed by his team in order to wear their vests and prepare themselves.

Of course, no one ever was really prepared for what was about to follow up. The dead body of the young man created an angry feeling in their stomachs especially in one's. In that exact same spot, he was fallen on the ground struggling to breathe, waiting for an imminent death that never came. The way Alex's body was put, his head, arms, legs, everything reminded of Jay. The same feeling of weakness and fear consumed his partner too.

"Do you think what I am thinking?" She turned to him.

"Why would someone replicate the scene, I don't understand."

"Maybe they are after you."

"Noo… Not everything is about me." Jay declared walking back in order to search for any leads.

After a lot of investigation, the whole team was back at the district comparing all the clues they had found so far. From the witnesses Kevin interrogated the offender turned out to be male, around 30s, wearing a police uniform. At first none of them could believe that a man from their job could have killed one of their own but soon the dots had started to connect and point at one exact person from the 21st.

"Mouse, could you run the footage from the surveillance cameras?" Voight demanded received a positive nod from the young soldier. "Halstead, Upton, my office. The rest of you continue working. And no word downstairs. We are gonna catch this son of the b*ch off guard."

Curiosity was written all over his face as Jay marched into his Sargeant's office with Hailey close to him. The latter closed the door and sat on the small couch opposite to the office while Jay stood there staring at both of them.

"You know I was sure I recognized the crime scene from somewhere… You got shot there, didn't you? The day that your father died."

"Did she tell you?" Jay pointed at his partner raising an eyebrow.

"She didn't have to… But she gave me further information like… The body…"

Before the older man finished his line, the young man cut him out. "No, stop. One of us was murdered and you still believe it was about me? Just because an angry woman shot me, it doesn't mean that the whole world is after me. Instead of arresting the shooter, we are sitting here making assumptions if he's after me or not. I can assure you that he's not. I get it, you try to protect me cause of a coincidence but this case is completely different. And you… You stay away from me."

Without saying another word, he slammed the door at his path, descended the stairs towards the locker rooms in order to calm down a bit. He hated upsetting his partner, he immediately regretted it, but he was also sick of them treating him like a baby. On his way to the room, a guy matching exactly the murderer's description accidentally hit his shoulder.

"Excuse me, detective." The boy said in a smiley expression.

"Don't worry."

"Any news on Jonas' killer?"

If you ever open a dictionary to the page where the word danger is, Jay Halstead should be a synonym to that adjective. In Afghanistan, every time a soldier seek danger, they dealt with the situation in their own way. That's what he did. The guy who killed his colleague was just standing there, laughing while a family was left alone grieving their member.

"I don't know, Belle, what do you think?"

The officer swallowed once before replying. "I hope you find him." Making a step, he was stopped by the detective.

"Cut the crap, okay? We both know that it was you. Why though? And why would you leave him in that position? Don't try to escape, they will get you in no time."

Belle stood still like his legs were pinned on the ground, unable to form any words.

"You know my team, well Voight and Hailey, my partner thinks that it's me you want. Is it?"

"You killed my brother."

"Your brother?"

"Daniel… You think I don't know what happened that day, how the vest saved you? He wasn't that lucky though, he didn't have protection."

"You brother murdered my father… I don't get it if you're after me, why Alex?"

"You needed a distraction."

An innocent boy was killed because of him. Guilty. That's how Jay felt at the moment.

"My plan was to come and kill you in the first place… Then I thought…. It would be nice to trick everyone for a while, to get to know them, to discover more about your personal life… Like that you have a brother…"

"Don't you dare to touch Will."

"That wasn't my purpose. I just wanted you dead." Officer Belle pulled out his gun of the holster in his waist and rang a shot which was easily dodged by Jay, however the guy took the advantage and ran outside.

Despite the efforts of the other officers in the door to stop him, the offender got out of the building running away being chased by a determined Jay Halstead. They both ran till they found themselves on the ground having been hit by a random driver who happened to cross the road at that time. No matter how much pain he felt on his chest, legs and arms, Jay literally jumped on the young officer, stabilizing him.

The loud bang didn't go unnoticed by the members of Intelligence who stuck their gazes out of the window, gasping at the sight of their friend and colleague.

"Sarge, Sarge." Hailey screamed at the top of her lungs speeding up to the interrogation room where Voight was currently at. "Jay had an accident."

"What?"

"Come on, follow me downstairs."

Almost all the workers of the building including Mouse exited it surrounding the two men. Belle was taken away by Platt as a woman called two ambulances at the scene.

"Jay, are you okay?" Mouse asked terrified, kneeled down to his friend who was sitting on the road holding his head.

"I am fine, just a little blood, don't worry."

"The ambo is five minutes away." Voight announced joining the two men in order to check his "man" out.

"No, no I don't want to go to the hospital, I am good." His first attempt to stand was successful so everyone considered that he was telling the truth indeed.

Jay lasted a couple of steps before collapsing on the ground and attracting the whole world again.

"Stay with me Jay, come on." Hailey begged, checking his pulse. "Don't you dare to leave me okay?" Suddenly she was pushed by someone and despite her protests, she stayed there.

Sylvie Brett and Emily Foster were in charge now to define his destiny till they reached Chicago Med.

The waiting room of the hospital was as busy as always. Lots of families waiting for news on their loved ones alongside the Chicago PD policemen and detectives looking forward to finding out Jay's fate. For most of them, the scenery was familiar, it happened eight weeks ago, for Greg, this was a prototype. Sitting there next to Hailey, he felt helpless.

"He's a fighter, you know." She said to him, in an effort to comfort him and herself.

"I know he is. But…"

"He hasn't changed a bit since you left him... Alright, maybe a down spiral but he pulled through. Okay and a shooting, actually two…" She chuckled.

"What you said about that moment, was he in pain when you found him?"

"Yeah, he was. I still can't believe that someone could copy a painful memory only to get revenge."

"I wish I was there when his father died. I didn't even know he was hurt back then… I am his best friend; I should have been here for him. I should have saved him now like I did when we first came back when he started drinking and passing out every night. I brought him back then and I should have protected him now.

"You are back, the first person you came to, was him and that's all that matters. Don't break down, it's doing more harm than good. Do you want to get some air?"

He nodded standing up looking at the ground. She grabbed his arm gently leading him outside the ER to a bench located next to the ambulance bay.

The sky was still crystal clear the sun was up and shining. None of them knew how much time had passed since they were have gotten out until the figure of Will Halstead appeared in front of them. "He's fine, he will be. The motherf… scared us there for a while but he survived. His ankle is sprained, his left shoulder is dislocated and he suffered hem thorax." Will announced blowing all the air out of his lungs.

"Can we see him?" They asked in unison.

"Sure, he's in the ICU now, we are gonna try and keep him there as much as we are able to. Follow me."

The pair made its entrance in the emergency department receiving smiley glances from the staff and their friends. For Mouse, it fell like hours before the elevator reached the destined department, for Hailey on the other hand, it was a habit walking to the ICU in order to visit her "man". Thus, she held his shaken hand smiling, as they followed the older Halstead to a spacious room with a bed located inside and around it various machines hooked up to the detective.

"We replaced the oxygen mask with a cannula because the first thing he did in the recovery was to remove it. Please make sure that it stays under his nose, his Sp02 is not 100% yet. Also try to keep him calm cause he is a little agitated." Will announced opening the door revealing his brother fighting a nurse.

"Hello there buddy."

In the hearing of Mouse's voice, Jay stopped complaining and turned his head to face the three people approaching him. "Hey guys."

"How are you feeling?" Hailey asked stroking his hair.

Mouse didn't say anything, he just observed the injuries on his body. It kinda reminded him of that day, after the convoy when he woke up and faced Jay in the same state.

"I wanna get out of here." Jay called, begging his partner.

"Yeah, there's no way." She replied taking a pick at the silent man.

"I'll leave you alone for some time."

If you ever asked Hailey Upton who her best friend would like to be, she would tell you, Greg Gerwitz. Even though, she knew him only for one day she had a hunch that this guy would change their lives for sure.

Hello, there loves! This is a one-shot that they suggested on Twitter and I decided to develop. Thanks for all your follows and favorites! Stay safe!

~The Fourth Bionic~