AUTHOR NOTE: This is an AU where Erin is actually Voight's daughter and a college senior. Everyone else are detectives as on the show. Jay is around 25 and Erin is 22.

He wasn't sure how exactly he ended up in this situation. He guessed it was partly wanting to impress his new boss and partly being the new guy in the office.

It was probably the latter.

Jay scratched the back of his head, adjusted his grip on the backpack slung over his right shoulder and took a deep breath.

"Here goes nothing," he mumbled to himself and headed towards the entrance to campus library.

Jay followed the signs towards the library and walked through the doors. It was huge and he had no idea where to start looking. He decided to head left but encountered a set of turnstiles.

"You need to swipe your card." The librarian at the reception desk told him in a tone that reminded him of Sargent Platt. Jay fished out his student ID from his jacket pocket and swiped; the turnstile light turned green and he pushed through the gate.

He noticed a sign that pointed to the Study Center and figured that it would be a good place to start.

The Study Center was located on the first floor of the library and, on a Tuesday morning, it was pretty empty. Jay could easily identify the blonde girl he'd seen a photo of in his case assignment folder. She was talking to a younger guy near one of the windows as they both hunched over a textbook.

He picked a table close enough that would allow him to listen in on their conversation but far enough not to draw suspicion and took out a textbook and pen.

"Just read this case study and tell me what you think the prosecution's argument might have been." Her voice was soft and husky. It was different to what he imagined but Jay realized that it made perfect sense.

After all – she was Voight's daughter. There was no mistaking that.

Jay forced himself to focus on the book in front of him – Greek Architecture and Sculpture – and pretended to be highlighting notes inside.

It was 10:47am which meant that the study session going on across from him would be coming to an end soon. That's when he needed to make the move.

It seemed strange that his first undercover assignment would be to be tailing the boss's daughter, but Jay had been the only detective Erin hadn't seen before and Voight had been pretty insistent that this was a special case.

They had spent the last three long months working on a complicated gang case in Intelligence. It had quickly consumed the entire unit and they had poured a lot of resources into the investigation. The team had successfully managed to infiltrate and take down a small gang that had previously dealt with the main target. It was an important step in getting ahead of the case.

Unfortunately, the leader of that small gang had threatened Voight's daughter and this was why Jay was currently sitting in a university library pretending to be a student.

As Jay understood it, Erin Voight wasn't a huge fan of police protection. Voight had attempted to assign a patrol car to park outside her apartment last week but she believed that a protection detail was a huge inconvenience and waste of police resources. Erin had sent them back to the precinct with that very message for her father.

Then Voight assigned Atwater to follow her home from a study session the next day. She recognized Kevin immediately and told her dad, in no uncertain terms, that she wasn't interested in being followed.

Olinsky tried to reassure Voight that Erin was perfectly capable of taking care of herself, but Voight didn't want to take the perp's threats lightly. This case had Voight rattled and stressed. So much time and effort had been invested in the case and he didn't need to be worrying about Erin's safety when the unit was so close to making huge progress.

That's when Olinsky came up with the new plan.

What if they sent someone undercover to protect Erin without telling her? Voight pointed out that they had tried that with Atwater trailing her but she had figured it out immediately.

"Obviously she did," Olinsky had told Voight, "because she knows Kevin."

"Also he's seven feet tall." Ruzek added helpfully.

"So what do you suggest?" Voight demanded.

"Get someone in Erin's life that she wouldn't suspect. She's a cop's daughter; of course she's going to be on the lookout for people following her. If you really want her protected, you need someone on the inside."

"I don't know many of Erin's friends that I would particularly trust." Voight shook his head.

"So get one of us." Ruzek said. Olinsky looked around the unit.

"Erin's met most of us," he pointed out. He looked at Ruzek and then shook his head. "No, she met you last summer at the barbeque."

"There's Jay." Ruzek said. "He's only been here three months. Erin hasn't met him yet, right?"

Voight considered it for a moment. "Jay, you know Erin?"

"Uh, your daughter? No." Jay shook his head. "I mean, I know of her, but I've never seen her."

This was true. He knew, of course, Voight had a daughter called Erin. Not that Voight had ever spoken of her that often. Jay had briefly seen her photo on Voight's desk and had been in the office a couple of times when she had called him on the phone. But he'd just heard the name and put the two things together. After all, he'd only been there three months and Voight was the stoic type.

He knew Atwater loved the Cubs and the Blackhawks. Jay went to Ruzek's place with Antonio most Sundays to watch football. He didn't know all that much about Olinsky but it was probably more than he knew about Voight's personal life.

"Hmmm," Voight didn't sound too confident about the new plan.

"Nadia?" Olinsky offered.

"Erin calls the office a lot. She knows Nadia's voice. They talk." Antonio added. "Boss, I think if you want to do this, Jay is going to be your best bet. He barely looks 25. He can definitely pass for a college senior or grad student."

Voight looked Jay over.

"This will be your first undercover assignment in this unit – do you think you can handle it?"

Jay nodded. "Yeah, no problem."

"It's only going to be until this case is squared away. Four weeks tops." Voight started writing something down on a piece of paper.

"Alvin, you think you can get Jay enrolled at the university?"

"Do we even need to?" Olinsky asked. "It seems unreasonable for a month-long undercover. We're almost halfway through the semester. I don't think I can get him in without some serious effort."

"Well, we need him on campus."

"He just needs a student ID. I can get that, no problem." Olinsky started flipping through contacts in his phone. "We should get him into a class if we can."

"Fine. Antonio, get me Erin's schedule. We'll figure out where we can get Jay in. Here's her tutoring days." Voight handed him the piece of paper he had been writing on. "Ruzek, you and Atwater can go scope out the university. I want to know where exactly Erin's classes are and where we can set up observation points for places Jay can't get into."

"Want us to talk to campus security?" Atwater asked.

"Not yet. Let's see if this is something we can pull off first." Voight then turned to Jay.

"You good with this, Halstead?" Jay nodded. "Good. You just need to befriend her, okay? Find out where she goes whenever she's not in class. Her classes might be safe but I want to limit the amount of time she's alone. I can set up an undercover patrol outside her apartment again but in case she notices them, I'll need you to be with her whenever you can." Voight patted Jay's shoulder. "You do well in this case and I'll make sure you get a good undercover assignment next time." He promised.

Something flew past Jay's head and it broke him out of his stupor. He turned to see Kevin groan.

"Missed you by this much," Atwater held his thumb and index finger close together.

"Sorry, Jay Ball isn't going to work today." Jay joked.

Kevin, Adam and Antonio had invented Jay Ball a few weeks ago. Jay had a habit of sitting pretty still; making him an easy target to throw objects at. It was one point to hit Jay's desk, three points for his body, and five points for his face. Ten points if you could get the object in his mouth.

Jay wasn't a huge fan of the game but the rest of the team seemed to like it. Even Nadia had joined in a couple of times. However, these games were usually reserved for when Olinsky and Voight were out of the office.

"Not when you're a moving target," Kevin said as he tore off another post-it and scrunched it up. "Hold still." Jay dodged the next post-it just as Ruzek entered the office.

"Good news, bud," Adam said as he tossed a folder onto Jay's desk. "Erin's taking a Greek Architecture class and it's a big lecture hall. Olinsky said we could get you in since there are so many people in it. The prof doesn't take a register either."

"Greek architecture?" Jay made a face. "Do you know this stuff?"

Adam snorted. "No way, man. Why do you think I became a cop? I barely graduated high school."

"Speak for yourself, Ruzek. I got a 3.6 GPA." Kevin pumped his fists in the air. "That's the exact GPA in your new transcripts too." Jay wasn't fully enrolling but Olinsky created some paperwork on the off-chance someone looked into his presence at the university.

"Why can't you go undercover instead?" Jay asked Kevin.

"Erin's seen me. She knows I work with her dad," he shrugged in reply. "Plus it's not like people would buy me as a student. I'm almost 30 and look like an undercover."

"I thought you couldn't wait to get your first undercover assignment in this unit, Jay." Ruzek asked.

"I want to go undercover but I didn't think it would require studying." Jay flipped through the study outline for the Greek Architecture class. "I know nothing about Greek pottery, Ruzek."

"Erin's in this class, dork." Adam retorted. "You're meant to be taking the same classes as her."

"Can't you find another class she's in? Or pick some bullshit class like Bowling 101."

"Nobody has a bowling class." Adam rolled his eyes; something Jay noticed he had a habit of doing.

"Have you ever been to college?" Jay asked. "I didn't think so. Just pick some Phys Ed classes."

"You're meant to be a senior. You can't take easy classes. Erin doesn't even take Phys Ed. She's a Criminology major. I can't get you into them because we're almost halfway through the semester and she's taking some hard classes." Adam pointed out. "Look, don't worry about it. You're only there for a couple of weeks until this case is solved so it's not a big deal."

"Anyway, you just need an excuse to talk to her. The class is just a way to make her believe you're not a cop." Kevin reasoned. "Just be glad Olinsky couldn't enroll you because midterms are coming up and you're definitely behind on the reading."

Nadia came up to Jay's desk with a box. "This just got delivered."

Kevin opened up the box and laughed. He pulled out the thick textbook for the class and handed it to Jay. "You better start reading."

So that's why Jay was currently looking over a Greek architecture book and trying to remember the differences between ionic, doric and Corinthian column types.

"I think that's enough for today, Kenny." He heard Erin's voice followed by the sound of a book closing. "Let me know if you want me to look over the first draft of your paper. I have a paper due tomorrow but I'll make sure to look over yours later in the week."

"Thanks, Erin," the kid replied. Jay watched from the corner of his eye. Kenny packed up first and waved goodbye to Erin as he left. Erin was still packing away a couple of things.

It was now or never. Jay took a deep breath.

"Hey, you're in Greek Architecture and Sculpture with Professor Robertson, right?" Jay turned in his chair. "I thought I recognized your voice."

Erin looked over at him. "Yeah, I'm in that class."

"I'm Jay. Jason Moore." Jay extended a hand towards Erin. She hesitated before shaking it.

"I'm Erin Lindsay," she replied. She squinted at him. "I'm sorry, but I don't think I've noticed you in the class before."

Jay shrugged. "It's a big class. I tend to stick mostly to the back. I'm not super great on being on time… or there at all." Erin let out a small chuckle.

"Yeah, I guess it's not really a class where you get to speak to a lot of other people. Robertson always has a lot to say about temples and pots."

"Are you headed there now?" Jay asked. "It's at 11:30, right?" He looked at his watch. It was 11.09am.

Erin looked at her own watch. "Sure, but I've got a little time. I was going to head down to the juice bar and get a smoothie." She told him. There was a short pause. "You wanna come?"

Jay nodded. "Sure, that'd be great." he piled his stuff, and that ridiculously big textbook, into his backpack and followed Erin out of the Study Center.

In a car parked across the road from the library, Ruzek spoke into his radio.

"Jump Street and Elle Woods are on the move."

"Copy that." Atwater's voice crackled from the other end. "Also, Ruzek, you suck at picking code names."