The trick is, once you're inside, walk like you own the place. That plus some judiciously forged documents will get you pretty far.

All the way to where River's room should be, even.

The doors on the hall - well spaced apart, never two across from each other - are made of a cool, smooth plastic pretending to be glass, set firmly into their frames. They're opaque, frosted, no indication of whether there's a light on inside or any inhabitants. No labels on the doors. No consistency, even, to the internal layout. No maps for emergency evacuation. There's a keypad hidden in the wall by each door - Ellie had to forge quite a bit more than just documents to get a physical key, a password, and then her biometrics into the system without raising any flags. The biometric scanners can detect signs of nervousness, she was warned. They won't let anyone who doesn't seem perfectly calm, sober, and alert through. 

The hallways are dim, lights behind thick shatterproof glass, with frequent doors of the same material as the room doors requiring her to scan through. She's seen one of the rooms checked, in passing - the door can be turned until transparent, one way glass. They're incredibly, harshly bright inside, and the workers examining them wear mirrored glasses. (She didn't see or hear the room's occupant.) There's no cameras, though. The scanners on the doors report access, but on a randomized delay - usually short, but long enough to be very strange for what's clearly a high security facility. 

Ellie hears nothing but the tap of her own boots, the beating of her own heart, when she isn't near the staff, and what little noise there is doesn't echo like it should. She hasn't seen a single vent, and the air feels stale, thin.

(The people who helped her in profess no knowledge of what's going on in here. It's a black box, and so far anyone who's gotten in has just vanished. They didn't tell her much at all, because of that. No sense in giving a dead woman walking the potential to leak information, is there? But a desperate sister is convenient enough, and if Ellie survives, they have indicated they'd appreciate information.)

Here's River's room, indistinguishable from the others. Ellie knows it only because she has been counting turns and doors. The keypad is in the same location as the rest have been, and - hopefully - the system will keep assuming Ellie belongs here. 

(She knows it because it feels right, like a familiar whisper, a gut instinct that led her along the tenuous chain all the way here. She knows it the same odd way River's letters leapt out at her in a code no computer could break. It whispers help.)

She slides the keycard through the scanner, punches in the code-

-takes a moment to steady her breath, still her heart-

-places her palm on the scanner.

The door goes transparent. 

And in the blinding white room - 

Ellie's little sister, strapped into a chair, blindfolded, a mask over her mouth - head turned towards Ellie as much as she can with the chair facing sideways from the door. Her body language feels happy, though. Something relieved in the line of her shoulders.

Ellie's contacts didn't know how to get the 'students' out of the rooms, once you get this far. Ellie can tell, though - it feels obvious, which new sequence will open the door.

She can't react just yet because she still needs to fool the scanners.

Ellie impatiently inputs the code to open the door.

It slides open. 

There's a nicely obvious catch on the straps, tucked away behind the chair. River's tense. 

"River, it's me," she says, striding quickly into the room and around the chair. "I got your letters." She releases the catch for the straps, fiddles with the gag, the blindfold.

They come off.

"Knew you would," River mutters, voice cracking, expression distant and hazy, almost stunned. "You always will."

"Always." She moves around to the front of the chair. "Always, always. Can you stand?"

"...Maybe. Is fuzzy. Stand - we'll stand - " She squints. "...Wrong planet, where we're standing."

"We need to - leave, don't we?"

She is going to come back with explosives and tear this building down. She is going to hunt down everyone involved and kill them slowly, with a knife.

"Yeah, River. We need to leave. Here, take my hand."

She takes Ellie's hand. 

"A sniper rifle will work better than a knife," she tells her sister, solemnly. "They'll be dangerous to get close to."

-she didn't say that out loud, she thought-

"But I want to see them bleed." She pulls River up, hugs her close.

"I'm sorry I didn't come sooner."

Snuggle. 

"It doesn't work sooner," she says, sounding exhausted. "I don't know enough, sooner." She turns her head into Ellie's shoulder, crying. 

Ellie pats the back of her head. "Shh, shh. I'm going to get you out of here now, okay?"

"Yeah. Okay."

"All right. Let's go." Ellie will mostly carry River out, if she needs it.

River needs only a little help walking. 

- And she starts issuing instructions to Ellie, though they're sometimes garbled. Right here. Wait here. Left - walk quickly...

Ellie will trust her sister's guidance.

They seem to be passing very near people a lot, especially towards the end - River drags them through a kitchen in a brief quiet spell between people, someone coincidentally facing away - 

And, finally, a fire door. 

"It screams," River whispers. "But quietly. But there's no people. We can run."

"Stay close." Open the door, and out.

She clings to Ellie, giving rapid - but increasingly fragmented - directions, heading towards a part of the city that's less well monitored - and there's a commotion behind them, someone yelling -

"Time to run now. Ready?"

"Ready," she says, a bit wavery.