This is a pretty busy episode, so let us proceed, shall we?
The episodes opens up with Zelenka grumpily packing up gear and preparing to head offworld. We quickly learn he is headed to Kid Planet to effect some repairs on their EM field. He is grumpy because he does not care for children and he's pretty sure that is exactly why Rodney picked him for the mission. Sheppard bids him good luck and then heads on up to the control room for a briefing with Rodney and Elizabeth. Rodney informs them that the city's sensors have picked up two wraith cruisers in the vicinity of their planet. The ships do not appear to actually be headed toward Atlantis, but Elizabeth wants him to continue to monitor their progress, just in case. Rodney also notes that he has picked up intense bursts of radiation between the two ships. It appears as if they are fighting one another. Everyone gloats and hopes the wraith all just end up killing themselves and saving the rest of the galaxy the trouble.
We then jump to the Cheyenne Mountain facility on Earth, where Agent Barrett of the NID (an offshoot of the FBI that deals with super secret departments like the SGC) has arrived to inform General Landry of a problem. A rogue branch of the NID, the Trust (which aims to gain access to alien technology without following protocol or diplomatic niceties) has apparently infiltrated the government even further than we thought. Barrett has learned that not only have the Trust infiltrated the SGC, they've managed to get an operative to Atlantis. It gets worse, Barrett goes on to tell Landry that their intel has revealed that the Trust has planted a bomb somewhere on Atlantis and it is set to detonate the next time Atlantis dials in to Earth for their weekly progress report. The next dial in is scheduled for today. Landry brings in Dr. Bill Lee to get to work immediately on a way to get word to Atlantis not to dial in to Earth. They can't use the gate because Earth doesn't have a ZPM and so no capability to dial an extra-galactic address, and it is possible that dialing in from Earth is equally likely to detonate the bomb.
On Atlantis Rodney is both surprised and creeped out to learn that Cadman is still in the city. She had apparently arrived on the Daedalus with its last visit and was supposed to leave with it as well. She tells Rodney that something came up so she asked Sheppard if she could stay a while longer and Sheppard agrees. Rodney is less than pleased and Sheppard ribs him a bit about being so uncomfortable around Cadman still.
Teyla, meanwhile, has learned that one of her people is very ill. Charin is like a grandmother to Teyla and Teyla asks Carson if he would be willing to visit the mainland to take a look at Charin to see if there is anything he can do for her. He agrees and they head out.
On Earth Dr. Lee has come up with a way to get in contact with Atlantis. The Daedalus, on its way back from the Pegasus galaxy, is currently too far away for the SGC to contact by normal means, but Dr. Lee says that if they gate a science team to a planet on the very edge of the Milky Way, the team should be able to reach the Daedalus with their most advanced long-range communications device. The Daedalus should still be in close enough range to contact Atlantis with her own long-range communications system and can then pass the message along. Dr. Lee likens it to the Twilight Bark from One Hundred and One Dalmatians and when he is met with nothing but blank stares at this switches his analogy to the signal towers from The Lord of the Rings movies, which everyone immediately gets.
They do manage to get their message to Colonel Caldwell, and he asks Novak if they are still close enough to relay the message to Atlantis. Novak replies that they are out of range, unfortunately, and Caldwell is in the process of telling her to alter the ship's course when Kavanagh chimes in that there is no way they will be able to get back in range in time to stop the dial in, which is scheduled for two hours from now. Hermiod then pipes up that he actually should be able to make some modifications to the hyperdrive that will allow them to travel fast enough to get in range in time. The process will not be without its risks, there is a chance they could burn out the ship's engines if they travel at that speed for too long, leaving them stranded between galaxies. Kavanagh is all gloom and doom about the plan but Hermiod is confident in his calculations and believes they will move into range of Atlantis before the ship is damaged. Caldwell tells him to do it.
On Atlantis everyone is preparing to dial in to Earth and they receive a transmission from the Daedalus at the last second, aborting the gate's dialing sequence in time. Rodney immediately disconnects the ZPM from the gate to prevent any accidental (or otherwise) dialing of Earth before they find and disable the bomb. Elizabeth decides to suspend gate travel until the situation is resolved (leaving Zelenka stuck on Kid Planet for the time being). Elizabeth tells them that it is entirely likely that it is the goa'uld (Stargate SG-1's big bad) behind the Trust's attack, since they have recently infiltrated that organization. They immediately set to work trying to find and disable the bomb, as well as trying to determine who in the city might be the Trust operative.
On the mainland, it is clear that Charin is very ill. She doesn't seem upset, however, when she tells Teyla that she is dying. She seems very at peace with her fate, and she asks Teyla to prepare a Ring Ceremony. Carson asks what this is and Teyla explains that because of the wraith it is extremely rare among her people for someone to die of natural causes/old age, and when they do, such a thing is to be celebrated. The Ring Ceremony does just that, honoring the life the person has lived. While describing it to Carson, Teyla starts to break down a bit, telling Charin she is not yet ready to let her go.
Back on Earth Barrett updates Landry. He has learned that the detonator for the bomb is not on Earth (to be activated once the gate connection between the two planets has been established) as they thought. The detonator is actually somewhere on Atlantis. Landry has the information relayed to the Daedalus. Caldwell passes the information on to Elizabeth. They haven't had much luck finding the bomb yet, but are glad of any new information. Elizabeth tells Rodney to start going over the city's systems to see if he can find anything, and Sheppard recommends that he have Cadman help him, since she is an explosives expert. Elizabeth also sends a message to Caldwell asking the Daedalus to return to the city to help with the investigation (and because she believes it is possible the Trust operative might have decided to flee the city when the ship left).
The gate begins to dial by itself, trying to make a connection to Earth. With the ZPM disconnected, it is unable to actually complete the sequence. Everyone takes (brief) breath of relief and Rodney is explaining that the Trust operative must have hacked the city's systems to make the gate dial automatically in case the regular dial in didn't happen as scheduled. Elizabeth is congratulating him on thinking to disconnect the ZPM when Cadman calls out that the city has started transmitting a distress beacon. They manage to shut it down, but the damage has been done. The nearby wraith cruisers detected the beacon and have changed course. They are now heading straight for Atlantis. The Lanteans head to the mainland to evacuate the Athosians to the city so that they will be hidden from the wraith under the city's cloak when the cruisers arrive. While Teyla is getting ready to go, Carson pulls her aside and says that after reviewing his checkup of Charin, he has found a few things he can do for her to prolong her life and make her feel better. Teyla thanks Carson and says she will let Charin know.
Caldwell has the Daedalus return to Atlantis so that they can help with the investigation into the Trust operative. There is a brief scene on Earth where Walter reports this information to Landry, who is less than happy, and says that there is nothing more they can do to help Atlantis in this situation. They are on their own. Elizabeth, meanwhile, has been going over passenger manifests from the Daedalus and has unfortunately found a few potential candidates to be the operative. She hates suspecting one of her own people, but she begins the process of interviewing likely suspects. Rodney pipes up that they should put Cadman on that list but both Sheppard and Elizabeth scoff at the idea, dismissing Rodney's suspicions to his dislike of the officer. Elizabeth's list is actually topped by Kavanagh and as she questions him, he bristles and responds with his usual foul temper.
On the mainland, most of the Athosians have been evacuated and Teyla is staying with Charin until a medical team can move her. She tells Charin of Carson's offer of help, that he can prolong her life, but Charin shakes her head and declines. She tells Teyla she has lived a full life and it is her time, she will not fight this and asks that Teyla not either. Teyla then breaks down completely, telling Charin that she is the only family Teyla has left and that if she dies, Teyla will then be truly alone. Charin tells Teyla that her people are her family and they will be looking to her for strength in the coming days. She will not be alone but she must be strong for her people.
In addition to questioning people she suspects of being the operative, Elizabeth also questions more trusted members of the Daedalus crew to see if any of them noticed anything suspicious or at least unusual. Novak tells Elizabeth that that "pony-tailed guy" (Kavanagh) seemed to always be up in everybody's business and has been kind of creepy, seemingly confirming Elizabeth's suspicions.
Rodney briefs the senior staff on their progress with the bomb. It turns out there actually isn't one. That is to say, there is no actual explosive device planted in the city. The operative was smart and rewrote Atlantis' operating system to disable the failsafes in place to prevent the ZPM from overloading. This would make something like dialing Earth cause the ZPM to go into catastrophic overload, causing an explosion that would result in the city (and likely planet) being completely destroyed. Unfortunately, that isn't the only routine that could cause the ZPM to overload, only the quickest. When the dial in to Earth failed to set off the ZPM, the operative's program used the automatic dialing attempt to distract everyone long enough to send the distress beacon, attracting the wraith. The ZPM has to be hooked back up in order to power the cloak hiding the city's existence from the wraith, which means that the operative's program will have new chances to overload the system causing the city to go boom.
All of this means they must find the Trust operative immediately. The wraith are on the way and the city has to be cloaked, there is no way around that. The failsafes can be reenabled, but it will require an access code from the operative to do so. Rodney says they don't have the kind of time needed to figure out the code on their own. Convinced that Kavanagh is the traitor, Elizabeth begins questioning him anew, but he maintains his innocence and that he has no code to give her. Ronon says they are wasting time and that he could get the code from Kavanagh in less than ten minutes, implying he would torture it out of him. Elizabeth and Rodney both balk at the idea of escalating to torture but both Sheppard (grudgingly) and Caldwell agree that it might be necessary.
Rodney waits until the wraith are as close as possible before reinstalling the ZPM and turning on the city's cloak. He and Cadman are standing by to monitor the city's systems for any sign of impending overload. It doesn't take long. The city's star drive (remember it is actually just a giant space ship) starts to gear up, and one ZPM is nowhere near enough to handle that kind of process. Without the failsafes, Rodney says they have thirty minutes until the city goes boom. Elizabeth and Caldwell start loading people into the Daedalus and puddle jumpers to evacuate the city and she also authorizes Ronon to do what he needs to in order to get the code from Kavanagh.
Charin, meanwhile, has passed on. Deciding to honor her wishes, Teyla and some of the other Athosians set up her Ring Ceremony in the infirmary. Carson is freaking out and trying to tell her she needs to get out. She explains very calmly that she and her group will leave on the final puddle jumper and then begins the ceremony. She sings a ritual song as we go into a montage of preparations to evacuate and attempts to stop the overload. Cadman finds Rodney and tells him she has found something while doing an analysis of the city's systems. Sheppard runs into the room where Ronon has Kavanagh yelling for him to stop. He finds Ronon standing over Kavanagh's prone body and is horrified until Ronon states that he hadn't done anything. Kavanagh fainted before Ronon could even touch him.
Caldwell is in his control chair aboard the Daedalus and radios to Elizabeth that he is ready to leave. When he doesn't receive a response he tries contacting her again and is beamed off the ship into the conference room on Atlantis to find Elizabeth, Ronon, and Sheppard. They tell him they found the evidence of his meddling in the systems and know that he is the Trust's operative. They demand the code from him and he refuses, his eyes glowing gold as he reveals that he isn't just a Trust operative, he is actually a goa'uld. Ronon knocks him down and Sheppard hits him with a taser, managing to subdue the goa'uld long enough for Caldwell's consciousness to surface and give them the code. They manage to get it to Rodney just in time and the city's failsafes are restored before the ZPM can overload.
As things wrap up, we learn that Hermiod is going to use the Asgard beaming technology to extract the goa'uld from Caldwell. Elizabeth asks Beckett to check out everyone else on the base and Daedalus crew just to be on the safe side. She is clearly very shaken by the whole situation. She is also beating herself up for allowing things to escalate with Kavanagh. Sheppard tries to reassure her that at least he wasn't hurt, but she is not comforted. She remarks on how they have been gloating over the infighting among the wraith and asks Sheppard how they are really any different.
Wow, so this is a pretty tight go-go-go episode. A lot going on. Yet they still managed to squeeze in a few nice character moments. We get Zelenka's dislike of children (likely exacerbated by this trip, since he gets stuck on that planet longer than intended and comes back having gotten a lovely new Kid Planet makeover). There's Cadman and Rodney's relationship in the wake of the events of Duet, as well as a budding romance between Cadman and Carson (which is the likely "something that came up" to keep her in Atlantis longer). Sadly, this is the last time we ever see Cadman on the series (I think this has more to do with Jamie Ray Newman getting busy appearing on like every show ever around this time than anything else), though the character does appear in a few of Fandemonium's licensed tie-in novels at least. We also get Kavanagh pretty much still being an ass-hat even though it gets him suspected of trying to blow up the city and refusing to cooperate with anyone he thinks is beneath him, which is everyone. This pretty much seals his fate about not being allowed back (and probably about not wanting to go back) to Atlantis.
There was also a lot of Teyla development going on. She is losing the last person among her people that she truly considers family. Charin is convinced that Teyla is meant to lead her people, even while she stays with the Lanteans, and urges her to not lose sight of that. This is a pretty good way of showing the audience how difficult it must have been for Teyla to throw in her lot with Atlantis and separate herself from her own people, as well as why it might have seemed like no big thing on the outside. She is very much accustomed (and expected) to projecting strength for her people's sake. Also, wow, yeah, that is totally Rachel Luttrell actually singing there. That woman is talented. I kind of wish they had done a soundtrack beyond just the first season so I could get her song, but alas, didn't happen.
The goa'uld back story to this episode was a bit confusing for me the first time I watched it, because that is all totally tied up to Stargate SG-1 and what was going on in seasons eight and nine of that series. "Critical Mass" actually aired on the night that I first watched SG-1 ("Ripple Effect") as well, and I will always remember how I was a bit lost while watching it. Still, even though viewers of Atlantis only might be a bit confused with some of the characters and information just dropped in from SG-1, watching it now, I think they did a pretty good job of only giving us what we needed to know to follow the rest of the episode. The goa'uld threat in Atlantis pretty much begins and ends with the possession of Caldwell and they are really only ever mentioned again in direct reference to this event or in passing in regards to events of the other series that have an effect on the people in Atlantis.
Caldwell being possessed by a goa'uld actually goes a long way to explain why he has been such a jerk since he got to Atlantis, as well as why he was always so determined to step in and try to take over whenever possible. I have a theory (completely unfounded and unsupported) that the writers either didn't intend to make him so unlikable or didn't realize how often he would be recurring on the show. Once it became apparent, however, they needed to be able to give him at least a little bit of a reset button so that the audience might have a chance not to loathe him for trying to mess with the status quo on Atlantis.
Novak's nervous hiccups are also a carryover from SG-1, I believe from a season eight episode. They are amusing even if you don't know that (well, to me anyway), but as a running joke it just makes me giggle even more. Though I love Novak oodles as it is. I would have heartily enjoyed a spin-off series about her and Hermiod, truth be told.
Lastly, I always get a kick out of Dr. Lee's appearance in this episode. Enough that when I go back and watch his first couple of appearances on SG-1 I cringe a little, because he didn't come off so well at first. Still, I love his little drawings depicting the message relay process, as well as the fact that he likened it to the Twilight Bark. I am not sure I buy that no one in the conference room would have gotten that reference, though. One Hundred and One Dalmatians was an old movie when I was a kid, so most of those people would have, presumably, been exposed to it at some point in their youths. Still, I always giggle when they do all immediately grasp The Lord of the Rings reference instead. So there you go.
"You'll find another way to get the message to them." (Landry)
"I will?" (Lee)
"Course you will. It's what I pay you for." (Landry)
"I would think after all this time you'd stop being creeped out by her." (Sheppard)
"She just has a way of getting under my skin. Literally." (Rodney)
"Dr. Kavanagh." (Hermiod)
"Stop talking, please." (Hermiod)
"Explosives expert, huh?" (Rodney)
"High temperature and energetic materials technology. And I can tap dance too." (Cadman)
We'll wrap up here, kiddos. See you back here on Wednesday for a slightly quieter episode, "Grace Under Pressure."