A/n: Welcome back one and all to part five. We're back to Mabel now because why not? Featuring even more OCs, but I promise they won't be too important. Reviews are always welcome and greatly appreciated. Sit back and I hope you enjoy.

Disclaimer: If I owned Gravity Falls, there would definitely be a season 3.

Operation Billwatch was a go. I just needed to find the right people for the job. Ally can be my reconnaissance agent since she knows how to get information through her immense gossip networks. Georgie can be the techie-nerd guy who knows all the weird, obscure things like Dipper...

Jackson would definitely be involved. The athletic one of the group. The romantic love interest that'll get with the main character after a job well done.

But first thing's first: Operation Billwatch. Objective: find out what is wrong with my brother's friend. Secondary objective: get rid of said friend if he's actually Triangle Bill.

How to get rid of Bill if it actually is that Bill?

Well, I'll deal with it if it comes to that. Hopefully, it won't.

But just the thought of Bill riffling through my brother's mind and turning him against me is enough to make me want to punch him in the face. Even if he's not demon Bill, he still is pulling Dipper away from me; for Pete's sake, Bill insulted me to my face and Dipper didn't say anything. Imagine what lies about me he's telling Dipper behind my back. Makes my blood boil.

Ah, there's Georgie, right in the back of the classroom like he normally is. Time to recruit the tech guy.

"Hey, Georgie, want to help me stalk the new kid? It'll be fun."

"Yeah sure, why not? But tell me why you have it out for this guy. The man just got here."

"He's really weird; did you see what he did to Johnny during lunch yesterday? What normal guy his size can do that?"

"My sixth sense tells there's more to the story than just that. Spill the beans."

"Fine, fine. I don't like how close he is to my brother. I don't think he talks to anyone else, and it seems suspicious to me."

"Your brother? Wait, that weird guy who'd show up and try to talk to you way back when. The kid even more socially awkward than I am? That brother?"

I nodded.

"Well, isn't good he finally has a friend of his own? I don't really see why that's a bad thing."

"It's my job as the Alpha Twin to look out for him. I don't want some creep to take advantage of him. Don't tell me you're not going to help me now?"

"Nah, nah, I'll do it. Just wanted to know why is all. Who else are you going to get in on it? Can't be just me."

"I'm thinking Ally and Jackson. I'm assigning you internet duty. If he makes a post, if he has Chirper, Instachat, or Snapgram. I want to know all about his internet history, if he even has one..."

"Who on earth doesn't have an internet history in this day and age? But, alright. I got you. It'll take me a day or so, though."

"Great, thanks. I'm going to get more recruits."

The next class was science. Ugh. Leave all that nerdy, dorky science stuff for Dipper, thanks. How could Dipper get through all this physics stuff and not burst into tears, never mind actually enjoy it. Then came gym, the only good class. I saw Dipper walking around the track with Bill. No surprise there. If I didn't know any better, I'd say they were attached to the hip. It's almost like how close Dipper and I were...

After last period, I rushed to see Ally before first trip left; she was on bus 15, so she left before me. Ally hangs out by her locker before she leaves. Locker 674.

"Hey Ally, I'm going to ask you to do something I think you'd like."

"What is it? Make it quick. I have to go soon, or I'll miss my bus."

"Would you mind asking around about Bill?"

"You think I haven't been? But there's nothing. Nothing concrete, at any rate. Why you want to know?"

"I don't like how my brother is the only one he talks to."

"I hear you. Every time I try to strike up a conversation, he rudely blows me off only to go talk to your antisocial lookalike. Don't worry, I'm in with your plan, whatever it is. I'll start digging deeper with the teachers. I'll let you know if I find out anything. I hope I find some juicy gossip. I gotta go now, but I'll see you later."

Ally headed for the doors. Another agent for Operation Billwatch. Now for the third and final recruit.

Jackson was the perfect guy: smart, ripped, and handsome. He managed to get Bs even in Ms. Gunderson's class, and that woman hates everything that lives; nobody, and I mean nobody, has ever gotten an A in her awful computer class. The closest most people get is a B-. I wonder if she's actually a witch. I wonder if Dipper would know.

Dipper...

Jackson often played soccer after school with the some guys from his neighborhood. He's pretty good; the last time I saw him play soccer, he scored three goals. Maybe he's near the field. The one behind the school. Mom and Dad won't be too mad if I stayed just a few minutes later to talk to Jackson.

Wait a minute! Is that Dipper near the exit? Must investigate further.

Oh my God, it is him! He hasn't left on time since the first week. It's a miracle! Maybe Mom and Dad will stop grounding him now, and I can talk to him at home again.

And he's with Bill again. Are they walking home together? Where does Bill live? Does he live nearby? Does Bill even have a home? Now would be a great chance to follow them and see what's up. For Dipper's sake.

But, doing that would mean I couldn't talk to Jackson today...

I'll leave them be, for now. I gotta go find Jackson. But hey, if Dipper doesn't come home today, I know who to blame.

Now, Jackson should be around the corner... Ah, there he is! Talking to Ben and Greg. Great!

"Hey, Jackson. Mind if I talk to you for a little bit? In private."

"Alright. See you two next Monday! And Greg, good luck with Tina, you magnificent bastard!" Jackson's friends walked away, and we were alone. Perfect.

"What'ja need to talk about?"

"Have you heard of the new kid, Bill?" He nodded. "He's really suspicious. I mean, did you see what he did to Johnny? He-"

"Wait, Mabel. You can't seriously be suggesting he's some supernatural creature or some shit like that; I thought that was your brother's thing."

Shit. Shit. Shit. Think of something Mabel.

"Uh no, I meant that he could be on steroids or some other illegal drugs. Stuff like that."

"That would make sense, true."

Alright, he's interested. "And that's not the only thing he does that bothers me. You know my brother, right?" Another nod. "Well, according to Ally, Bill talks to my brother."

"I guess that's a bit strange, but that's not really suspicious."

"No, no, I mean he only talks to my brother. Nobody else."

"Hmm... You may have a point. So, what do you want to do about it?"

"I'm thinking we should see what's he's up to. Investigate, like real detectives. I've already got Ally and Georgie in on it."

"Okay, sounds good. The group will meet up tomorrow, then?"

"Yeah."

"Okay. See you tomorrow then." Jackson walked off the way Ben and Greg went.

Recruitment mission success! Time to go home! Maybe Dipper will be back by then!

Ten minutes later, I reached our house. It wasn't as stylish or modern as Ally's house, but it was a good size. It was definitely better than that Mystery Shack I lived in over the summer.

"I'm home." My parents were sitting at the kitchen table. "Hi Mom, hi Dad!"

"Welcome back, sweetheart. Did you have a nice day?"

"Uh-huh. The English test we had today got postponed because the teacher caught the flu; we probably won't have it until next week, maybe more. Is Dipper back yet?"

Dad scowled. "No. He's probably at the library again. I swear, all that boy does is read study that strange occult stuff. All he does is stay inside with his nose in a book. He doesn't even talk to us anymore. Picking books over family; what kind of son did I raise? The other day I was trying to tell him to stop reading that weird novel and play outside with his friends, like a normal child, and he just plain ignored me. Back in my day-"

"Wait, what do you mean Dipper isn't home yet? I saw him leave school."

Did Dipper go off with Bill? Did Bill kidnap him or worse? Did something terrible happen?

"Relax, Mabel sweetie. Your brother probably went to a bookstore or another library, that's all. I'm sure he'll be back at the same time he normally is."

Maybe that's what Dipper would have done if he was alone. But he wasn't alone, now was he?

"No, no, you don't understand! He left with Bill!"

"Bill?"

"Dipper's new friend."

"Now, sweetie, it's a good thing Dipper finally made a friend. No need to worry."

"It's about time that boy socialized like the rest of society instead of sticking his head in a book 24/7," Dad chimed in.

"But Bill's creepy and weird and yesterday he punched a guy in the face so hard he hit the ground."

"And of course Dipper having a normal friend was too good to be true," Dad muttered.

"Now sweetie, you don't need to worry; it's probably nothing, just Dipper being Dipper. Just go upstairs and work on your homework, okay?"

What else could I do? I couldn't do anything to find Dipper now. I listened to Mom and hoped she was right.

Dipper normally came home around 3:30, yet he wasn't home. As the clock struck 4:00, he still wasn't home. He hadn't even come home by dinner time, so we ate without him. I was starting to get very, very worried; Dipper never stayed out this late before.

By 8:00, I was ready to call 9-1-1. Mom seemed worried too, by then, and even Dad seemed a little unnerved.

Finally, at 10:00, Dipper showed up. He seemed okay, thankfully. I was just glad he was safe.

"Where were you, young man!? Do you know what time it is!!??" Dad was definitely not happy. Even Mom looked a little put out.

"I was out with a friend, and we lost track of time. That's all," Dipper murmured, already heading for his room.

"Dipper, do you know how worried we were? How could you do that to us, never mind to your sister? How could you be so selfish!?"

Dipper stopped at that, turning around to look at Mom. His gaze was so unsettling. There was something... dark there, something evil and angry. It frightened me. It was like this wasn't my brother anymore.

And as quickly as it left, it was gone, and Dipper was back to normal. He went upstairs without another word.

What is happening to my brother?