Lee comes up with a plan to rescue Jeremiah and the others, as well as take out Valhalla Sector; but he needs Meaghan, and her virus, to carry it out.

Lee says he wasn't the only spy for Valhalla Sector at Thunder Mountain - there was a "sleeper agent" who would only emerge when necessary. He was right.

Thunder Mountain has an array of weaponry, including helicopters, that very few are aware of, mainly because of the lack of available fuel. However, Markus has ensured that the equipment was kept in working order, and that select residents were trained as pilots on simulators.

Lee plans to exchange Meaghan for Jeremiah, Markus, Erin and Devon. Valhalla Sector send only Markus and Erin to make the swap, but Lee has sent Kurdy to lead a frontal attack on Valhalla Sector to distract them. Valhalla Sector has the ability to shut down for a fixed period during which nothing can get in or out; Lee wants to pressure them into doing so, so that Meaghan can infect everyone inside and they can do nothing about it. He didn't expect them to keep Jeremiah and Devon, but as they are both locked in the cells they avoid being infected.

The base is locked down for 120 hours and the inhabitants including the President are infected and killed. The only survivors we see are Libby, Devon, Jeremiah and Meaghan herself.

Lee is known to Libby. He warns her about his plan and she takes food to Devon and Jeremiah, and kills their guard with home-made explosives. While the base is locked down she hides in a cupboard.

Meaghan commits suicide after infecting Valhalla Sector with the Big Death.

It would seem that everything Lee has said was the truth and his loyalties were firstly with Devon and secondly with Thunder Mountain. However, he may have been keeping his options open until he was in a position to decide who would have the upper hand in any conflict.

Meaghan's request to Kurdy: "Do me a favour. If Lee does betray me, you, Markus, and everything we've built here, for a second time - do you think you could, you know, sort of kill him for me?"

Mister Smith's reaction to a trooper saying he doesn't believe in God is "neither did I, but I don't think he noticed."

Waverly's last words: "We were going to r- run the whole… kings and queens of the new world… for the first time, no one could fight it. Nobody could stop us. We won, don't you see? A lot of people died, but we won. We won."

Meaghan, after infecting Valhalla Sector: "I made quite a… a mess out there" and "I'm scaring me to death."

The Valhalla Sector logo seen on a video screen is the same as that shown on Farralon's equipment ( "City of Roses" ); yet more evidence that Wily's information was right.

Lee gives us the fullest description yet of the effects of the Big Death: "The Big Death spreads by touch. It moves like wildfire. It starts with nausea, dizziness, vomiting, lesions and bruising around the joints. Then madness - dementia. Blindness. The internal organs liquefy. And then death. Total time involved two to three days."

Jeremiah's attempt to reach Waverly through the cell bars is mirrored later as Waverly tries to reach him.

Meaghan says to Markus, "You promised me I would feel the wind in my hair and the sun on my face." This is a reference to Season One's "Man of Iron, Woman Under Glass".

Was it the sleeper agent and not Lee who told Valhalla Sector of the meeting in St Louis?

Waverly was the first to be infected before the lockdown, but is still alive less than an hour before the lockdown ends, more than five days later. Perhaps some people are more resilient to the Big Death than others and take longer to die. Alternatively, although they may not have found a successful vaccine, Valhalla Sector may have developed drugs that would slow the progress of the disease and given them to their top people as a safeguard.

Additional - added 4 March 2006: It's been suggested that the odd timings of some events in this episode make more sense if the countdown says there are 35 hours left, not minutes - suggesting that the real nitpick is the speed with which the timer is counting down when we see it.

Is Meaghan's blood really that much use to Devon? Valhalla Sector must already have stockpiles of the Big Death virus, and Devon knows how to produce a vaccine. However, it might be worth investigating how she manages to carry the disease without it affecting her, as that might hold the key to a treatment for anyone not inoculated.

Meaghan's final scene is powerful stuff, but contains a couple of puzzling elements. Firstly, Meaghan is overstating how dangerous she is - Devon should be able to produce a vaccine to inoculate everyone she might come into contact with, and possibly even develop a cure. However, she may be saying this to hide the real reason for her suicide, which could be her inability to go on living knowing what she has done. But why does Markus not try to discuss this with her? Maybe because he suspects she won't be able to live with herself no matter what he says. Her method of suicide - jumping into a whirlpool - is a little suspect too: is she sure that she won't infect any living creatures in the water that might then spread the virus? It would have been better to return to Valhalla Sector to kill herself, thus allowing the Thunder Mountain gang to keep her body contained with the other infected bodies. Of course, her experiences (and the fact that she's lived in one room for fifteen years with regular contact with only one other person) might put such reasoning beyond her current mental state.

Was everyone inside Valhalla Sector killed? Were there no children, for example? The conclusion of the episode may elide the possibility that Valhalla was only decimated sufficient to remove it as a threat; certainly the President and anyone close to him is sure to have been killed. In which case, perhaps there were many, less important, survivors.

The Thunder Mountain gang manage to travel halfway across the continent and set up a decontamination base near Valhalla Sector within a few days. Even with Lee's inside knowledge, that seems a bit of a stretch.

Why make the swap-over point so much closer to Thunder Mountain than Valhalla Sector, rather than midway between them, when both groups have access to helicopters? Lee may have insisted on this in order to give Kurdy extra time to get to Valhalla Sector; but what excuse did he give to Waverly? It's a bit risky, too - the longer it takes for Waverly to get Meaghan back to Valhalla, the more likely it becomes that he will start to show symptoms of the disease before they get there.

An expansive, cinematic episode, laced with the blackest irony, and with no end of chilling scenes that leave an indelible impression on the viewer.