Halloween night at the FBI headquarters was never a dull affair, especially when the Behavioral Analysis Unit was involved.

The bullpen at Quantico was dimly lit, orange string lights draped across desks, and a few hastily carved pumpkins lined the conference table. The agents of the BAU, Hotch, Rossi, Morgan, Reid, JJ, and Garcia, were gathered around the briefing table, looking less like the elite team that hunted serial killers and more like a group of kids forced into a very strange office party.

At the head of the table stood Section Chief Erin Strauss, looking far too composed for someone delivering such an outrageous order.

“This comes directly from the Director,” Strauss said, arms folded. “Apparently, inter-agency cooperation is… lacking. So we’re to conduct a routine morale check on NCIS and CIA.”

Morgan raised a brow. “Morale check? On Halloween?”

“Precisely.” Strauss’ lips twitched — the closest thing to a smile anyone had ever seen from her. “You’re to go in costume. Specifically… zombies.”

Rossi chuckled. “We’re supposed to scare federal agents? You sure this isn’t a setup for a disciplinary hearing?”

“Think of it as… community outreach,” Strauss said dryly. “Now get going. And bring the intern.”

{{user}}, the team’s current intern, tried not to grin too widely from where they stood near the back. This was not what they’d expected when they signed up for field experience, but hey, how many people could say they got to prank NCIS and the CIA in one night.

Down in Washington D.C., Gibbs’ bullpen was unusually quiet for Halloween. Most of his team, Tony, Ziva, McGee, and Abby (who had already decorated her lab like a gothic wonderland), were buried in paperwork or reports.

Until the lights flickered. And then, silence.

Ziva glanced up. “McGee, did you—” Before she could finish, the elevator doors creaked open.

Out staggered Morgan, his shirt ripped and stained with fake blood. Reid followed, pale makeup accentuating his already gaunt features, dragging one leg behind him in a disturbingly convincing limp. JJ moaned like the undead, while Garcia, carrying a prop severed arm, shrieked in delight from behind them.

“Brains…” Reid groaned, stretching his arms toward McGee. Tony’s eyes widened. “Oh, nope. Not today, not the zombie apocalypse.” He vaulted over his desk.

“Zombies aren’t real, DiNozzo,” Gibbs muttered, though his hand was already on his gun. Then {{user}}, covered in expertly applied FX makeup (courtesy of Garcia), lunged from behind a filing cabinet with a “Boo!”.

Abby burst into the bullpen just in time to scream, and then laugh so hard she nearly doubled over. “Oh my God! You guys! This is amazing!”

Ziva blinked, slowly lowering her stapler (which she had raised like a weapon). “Wait. These are… FBI agents?”

Morgan smirked, peeling off a fake blood packet from his arm. “BAU, actually. Courtesy of Strauss. Happy Halloween.”

Tony groaned, shaking his head. “Only the FBI would weaponize zombies for fun.”

Rossi grinned. “Don’t get too comfortable. We’re heading to the CIA next.”

“CIA?” McGee asked. “You’re gonna get shot.”

Rossi shrugged. “Wouldn’t be the first

It was nearly midnight when both teams, BAU and NCIS, found themselves outside Langley, having apparently decided this was now a joint operation. Abby had tagged along, armed with a camcorder to “document history.”

Inside, the CIA’s glass-walled conference room was filled with agents and senior officials in the middle of a tense meeting about intelligence sharing.

That’s when the lights flickered again.

A low, echoing moan filled the hall.

The door burst open, and in came the horde.