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The Odyssey has brought SG-1 (sans Teal'c) to Atlantis in the Pegasus galaxy with a mission: to dial the Ori supergate in the Milky Way, thereby preventing the Ori from bringing any more ships into the Milky Way. Why go to the Pegasus galaxy to do this? SG-1 has been unable to dial the supergate from within the Milky Way, so they infer that the supergate can only be contacted from another galaxy.
SG-1, Dr. Weir and Col. Sheppard's team on Atlantis have a briefing to explain the plan. Dialing to the supergate in the Milky Way will take a tremendous amount of energy. And, keeping the supergate open for longer than the standard 38 minutes will require a lot more energy. So the plan is to place a stargate near a black hole in the Pegasus galaxy, and then use it to call a normal stargate that Teal'c is emplacing near the supergate in the Milky Way. Once the two gates have established a connection, a nuclear warhead exploded near the Pegasus's stargate will cause the wormhole to jump from the Milky Way stargate to the supergate. (The seed of this idea hearkens all the way back to season 2's "A Matter of Time" and was batted around again in last season's "Ripple Effect".)
The plan is tricky and will require a lot of computer simulations and complex calculations. Carter asks Dr. McKay to help them out on the Odyssey, which will be transporting them to the local black hole. While Carter, Mitchell, and McKay will be conducting this mission, Daniel and Vala will be embarking on a mission of their own. Daniel wants to use Atlantis's database to search for the other planets that Arthur allegedly visited in the search for the sangraal. The difficulty is that he knows the names of the planets in Old English, which is presumably not what the inhabitants of Atlantis called them.
The two missions play out separately, so I'll talk about one and then the other. Dr. Weir shows Daniel how to interface with the Atlantis database: it's pretty simple, because a holographic person appears and you simply talk to her. Daniel starts out with some basic questions, approaching his topic obliquely, but Vala is impatient to just ask directly about the planets. After all, it's just a hologram: if it can answer, great, but if it can't, no harm done. Daniel finally gives in and asks - and the hologram tells him the names of the planets in the Atlantis database so he can find out more about them.
This seems too good to be true, and Daniel realizes something is fishy. After some more questioning of the hologram, he realizes that the hologram is really an Ancient who is ascended. He deduces that she is the Ancient who appeared as Morgan Le Fey in Arthurian legend. Morgan finally gives in and admits it. She has been trying to help them out without getting in trouble with the other ascended Ancients. Daniel tries to convince her to help them more, but as she finally decides to tell them some crucial information, the other Ancients pull her away. (In nice continuity, this effect is the same as when Daniel was pulled away in "Full Circle" in season 6.)
In the other mission, Carter and McKay set up the first nuclear warheads to explode at just the right time. The complication is that if the yield of the explosion is too high, the stargate itself could be destroyed, which would foil their plan. Their first and second attempts are unsuccessful. Then an Ori ship arrives in the vicinity of the supergate, apparently just to check on it. Teal'c hides the normal-sized stargate in his ship's cloak.
Unbeknownst to the Odyssey, a Wraith hive ship has noticed their activities and is approaching. The Odyssey cannot detect them because of the effects of the proximity of the black hole, but Atlantis detected them and relayed a warning via the SGC and Teal'c. The Odyssey crew isn't yet ready for their final attempt, but if the Wraith find their stargate, they could destroy it. And if the Ori find Teal'c's stargate, they will realize what's up.
The Wraith attack, and Carter and McKay devise a plan to use the Wraith ship in their final attempt to "jump" the stargate. They will lure the Wraith ship into just the right position, and then beam the nuclear warheads onto the Wraith ship and detonate them at the right time to cause the stargate's wormhole to jump. They implement the plan and give Teal'c a twenty second warning to get out of the vicinity, because it's possible that both stargates will vaporize. But Teal'c is always cool and collected. He pauses and reveals his ship and stargate to the Ori ship at just the right time so that when the wormhole jumps and the supergate activates, the Ori ship is caught in the "whoosh" and is destroyed. Of course, the Wraith ship is also destroyed, so Earth scores a two-fer!
This episode was a lot of fun, and it has a lot of ramifications. First, though, seeing SG-1 in the Pegasus galaxy was a treat. It's very satisfying and realistic to see that even though SG-1 and Team Atlantis are on the same side, they have very different ways of interacting and operating and can get under each others' skins. In particular, the ways in which SG-1 tolerated McKay were amusing. Carter knows how to handle him, but Mitchell will not put up with any of his crap. I particularly enjoyed Sheppard giving Mitchell a lemon to use to motivate McKay.
The portrayal of the black hole was pretty good. I was initially a bit put off by the bluish coloring of the accretion disk around the black hole, but then I realized I was wrong. The accretion disk around a stellar-mass black hole would be millions of Kelvins, hot enough to be emitting primarily gamma-rays and x-rays. We can't see those types of light, but the way the light distribution is shifted toward the short wavelength light would definitely make the accretion disk look bluish. Good for them!
They didn't do quite as good of a job with the time dilation. They had voices over the radio being deeper than usual, trying to portray how light moving away from a black hole would be redshifted. However, there are a number of problems with their portrayal. First, radio light is not the same as sound! The radio signal carrying the information would have been redshifted when traveling from SG-1 to Teal'c, but that means that Teal'c's radio electronics might not have recognized the signal (because it's not the right wavelength any more). However, as long as Teal'c's radio correctly received the signal, the electronics in the radio would translate the signal into normal-sounding voice.
The second problem with this idea is that the signals in both directions should not have been redshifted. The signal is going from the Odyssey deeper into the gravity well to the stargate, then up out of the gravity well via the wormhole to Teal'c's stargate and Teal'c. So the light heading from the Odyssey to Teal'c is net going "uphill" and thus the signal to Teal'c should be redshifted. But that means that Teal'c's signal to the Odyssey is net going "downhill" and would actually be blueshifted. The blueshifting idea is something that was done correctly, as I'll discuss next.
When the Wraith are attacking the Odyssey, they are doing it from farther away from the black hole. Carter says that the Wraith "literally have the high ground", and they do! The Wraith are farther out of the gravity well than the Odyssey. If the weapons the ships shoot are light-based, as in lasers, then the Wraiths shots will travel "downhill" to the Odyssey and be blueshifted, which will make the light gain energy and be more destructive. The Odyssey would suffer the opposite effect: their lasers would be firing "uphill" and lose energy. Frankly, we don't know enough about the weapons to really know if everything worked out correctly, but overall I am pleased with the attention to the detail on the effects around the black hole.
What long-term effects will this episode have? A bunch! The Ori cannot bring more ships into the Milky Way unless the free up the supergate or make a new one. They also have lost a ship; from what we know, they are down to three ships. The forces for good in the Milky Way need to take advantage of these facts. But they still don't have an effective weapon against the Ori. As a bonus for Atlantis, one of the Wraith hive ships was destroyed.
On the downside, Daniel's experience with Morgan Le Fey tells us that the Ancients are not going to be any help, at least not yet. Will they actually let the humans in the Milky Way be wiped out or conquered just so they do not violate their principles? If the humans are neutralized, do the Ancients have any defense against the Ori? We have no idea. Will Morgan Le Fey be able to convince any of her kind to help, or has she been destroyed? And what was she going to say about Merlin's weapon just before she was pulled away? I have a feeling Merlin's weapon is not quite what our characters are envisioning.