
We see Weasel Shane attempt to move his friend, freaking out when he listens for a heartbeat but doesn't hear one. He has no choice but to abandon his friend, who he really actually truly thinks is dead. In other words, maybe Weasel Shane is not quite as much of a weasel as we thought. At least at this point, before he starts sleeping with Grimes' wife. Well played, show. Yet another character with several dimensions whom I now can't judge. Which is too bad, because I really love to judge others.
Shane blocks the door to Grimes' room and makes a tearful break for it, past zombies and soldiers.
Opening credits.

Before everyone gets too hammered to stand, Shane goes all buzzkill and asks Jenner exactly what the frak is going on with all these zombies, yo, and why isn't there a huge team of scientists filling this facility, buzzing away at finding a cure? Jenner says that everyone else either died of zombie flu, or left to be with their families, or just killed themselves before the virus had a chance to get them. So he's the only one left. Everyone bums hard.

The next morning, we get schooled in Zombie Making 101. After their first good night's sleep since the nightmare started, the survivors gather in the main room as Jenner drops some knowledge on them. He boots up a giant computer screen and shows x-ray footage of a person's head. Test Subject 19, he tells the others, someone who was infected by a Walker, and then volunteered to be examined and recorded for the subsequent transformation. Jenner lets the others watch as the x-ray head on the big monitor starts with a healthy brain, bristling with active synapses, and quickly turns black as the zombie infection takes over. "Death," Jenner says. "Everything you ever were, or ever will be. Gone."
Then everyone watches as tiny red sparks in the subject's brain stem start firing slowly. The brain remains dead, we learn, but the brain stem activates, the part that provokes mindless instinct. Andrea watches, thinking of her sister.
At the end of Subject 19's footage, something like lightning knifes through its brain; it's the point when Jenner had to shoot the subject in the head, of course. It doesn't come up until a little later in the episode, but if you're like me, you totally saw the reveal about Subject 19 coming: it's Jenner's wife. Sad.
To make everyone feel worse, Jenner says that there are no other scientists anywhere else. As far as he knows, everyone working on a cure for the virus is gone. Apparently the only scientists kept working on a cure during the zombiepocalypse were the French. Who knew? Way to go, French scientists! It's so weird that they’d be the ones to work so hard to save humanity, considering how rude they are to all other humans. But according to Jenner, even the brave and stalwart French eventually fell.
After that, everyone's sitting there, thinking, Well, crap. And the situation gets worse when Grandpa Dale asks Jenner what's up with the giant ticking clock in the back of the room. Jenner explains that the clock is counting down until the bunker runs out of power. And when that happens, the bunker's automated system will "decontaminate" the facility. Which is done by setting the oxygen in the building on fire, and exploding the building.
Wait… what?
Everyone panics. I panic on their behalf, even though this sudden dilemma is totally contrived and completely implausible, serving only to give us a big race to the finish in this season finale. But ok. Jenner has already sealed the inner doors to the facility, locking everyone inside. Dying in a huge instantaneous fireball is preferable to the slow shambling non-death by zombie bite that awaits them outside, he explains. He says there's no point in trying to survive out there. "This is our extinction event," he says plaintively.
There's a scuffling, wrestling, and both Shane and Redneck Darryl are keen on shoving a gun in Jenner's face to make him unseal the doors to the upper lobby. Their threats don't work, but Grimes' pleading does. After a quick "We deserve a chance to live" speech, Jenner unlocks the metal doors so our gang can head up to ground level and try to shatter the bulletproof glass in the lobby and escape.

There's some quick pathos – Andrea, still morose over the zombie-death of her sister, wants to stay and enjoy the fiery doom with Jenner. But it's Grandpa Dale who convinces her to make a break for it with him. Basically, he plays the "If you say, I stay" gambit, and it works. Andrea allows him to haul her up, and the two of them run to catch up with the others. Will they make it in time? (Yes.)
Up top, Grimes uses a grenade to blast a hole in the lobby windows, and they all climb out.

The group makes it to the vans. They turn and see Dale and Andrea climbing out of the building. Run Grandpa Dale! Run Andrea! Suddenly, I'm not totally sure you're going to make it, despite the fact that you're both listed as series regulars! After all, Andrea's sister Amy was a regular too, and she died.
Inside, we get a last shot of Jenner and Jacqui, another survivor who's basically been an extra up until now, sitting with him. She's decided to take the easy way out with Jenner – if you call dying in a massive building explosion easy.
Dale and Andrea make it to safety. The building goes boom in a massive display of CGI effects that look as real as it possibly can on a show made for basic cable. And as the group caravans off down the road in their vans, we pan up and follow the black, billowing smoke from the building, which takes on ominous shapes. And...
... credits.
Thanks, The Walking Dead! You were incredibly creepy and depressing and awesome. Get cranking on some new episodes and I'll meet you right back here.