[Leigh Butler, Drew Holton, Jennifer Liang]

The big cliffhanger of COT leaves us with several questions, but the central one is this: who ratted out Egwene's Sooper Sekrit Plan?

There are several possibilities, both for who the traitor was on the Rebel AS side and who captured Eg (and probably Leane too) on the Tower AS side (obviously, each influences the other).

There are also a couple of significant events preceding the operation that probably have something to do with the betrayal. One is the death of Kairen Stang [COT: 30, What the Oath Rod Can Do, 652], who was supposed to be Leane's counterpart in the undertaking. Another is Nicola Treehill's disappearance. Yet another is the unexplained absence of Faolain.

One thing to note at the outset: Egwene's captors are surprised at her identity. Some people have taken this to mean that the capture itself was unplanned, and the AS who captured her were just guarding the wall or something - i.e. there was no betrayal. However, this is highly improbable. The use of forkroot means that the ambush was almost certainly planned, rather than being merely fortuitious - as Ben Goodman points out, forkroot has to be brewed into a tea, which is kind of difficult to carry around "just in case". The precision of the abduction and the lack of a general alarm also suggest that they were forewarned of the plan. It's very unlikely Egwene's captors were there by happenstance; their surprise merely indicates that neither the traitor nor her collaborators on the other side knew about Egwene's last-minute decision to replace Bode.

Who knew about the plan?

Unfortunately, a whole lot of people. Egwene had informed the Hall of the scheme [COT: 30, What the Oath Rod Can Do, 666], so that means that not only did Romanda, Lelaine, Sheriam and the rest of the Sitters know about it, but so did Halima, via Delana (and also possibly Sheriam's torturer, if he/she isn't Halima or a Sitter to begin with).

Siuan, Leane, Bode, and Kairen Stang (before she died) also knew, obviously, since they were key players in the plot. Gareth Bryne was in on it [COT: 30, What the Oath Rod Can Do, 664], as was Theodrin; presumably the mysteriously absent Faolain was aware of the plan as well, though we can't know that for sure.

There is also a possibility that Nicola (and Areina) knew.

Which of these could be the traitor(s)?

A few of these people can be eliminated outright as suspects. Kairen, being dead and all, can be safely rejected. There's no way Siuan was responsible. Gareth Bryne, Leane and Bode are all extremely unlikely culprits as well. Let's look at the remaining suspects.

1. Nicola: She certainly seems to have been set up to look like she did it. Besides being a malcontent and a known blackmailer, she ran away from the Rebel camp three or four days before Egwene's capture, which would have given her plenty of time to get to the Tower and spill the beans about the plan. As to how she would know about the plan, Nicola has a history of ferreting out information she has no business knowing. It's also possible she learned about it through a Foretelling:
   

   [Tiana, to Egwene]: "Ever since we found out she has the Foretelling, she's been Foretelling two or three times a day, to hear her tell it. ... battles with the Seanchan or the Asha'man, an Amyrlin imprisoned..." [COT: 17, Secrets, 431, emphasis mine]

   It's been pointed out that Nicola doesn't remember what she Foretells, but Areina or her "family" were clearly telling her everything she said while tranced out, so that's no obstacle. Indeed that brings up another suspicious connection: Sheriam fiercely lobbies Egwene to let Nicola's "cousins" off the hook for covering up her escape. This could be just because Sheriam's own beatings have left her super-sensitive (so to speak) on the issue of corporeal punishment, but another way to look at it is that she is worried about what they might reveal under duress about either Nicola's escape or her other Foretellings, or both [Patrick Cotrona].

   Of course, it could also be that Nicola did run away to the Tower, but had nothing to do with the betrayal. She may have guessed what her "Amyrlin imprisoned" prophecy meant and simply decided to pick the winning side before it all went down. She could even have gone to the Tower in order to help Egwene, unlikely as that may seem.

    

2. Sheriam: Since she was revealed as a Darkfriend in TGS, she seems a very likely candidate. Maintaining the division between the Aes Sedai factions was a key goal of the Shadow. Such a decisive blow to end the siege peacefully would have been something Mesaana or Aran'gar could have ordered her to prevent. Also see the possible Nicola connection, above.

    

3. Lelaine: After having been so respectful for so long (if reluctantly) since the War Vote, the thinly veiled contempt Lelaine displays for Egwene at their meeting in COT is rather startling. She also makes an ominously ambiguous comment that seems very close to an open threat:
   

   "The White Tower makes mistakes upon ocasion. It is impossible to live or move without making mistakes. But we live and we go on, and if we sometimes need to conceal our mistakes, whenever possible, we rectify them. Even when it is painful."
   [COT: 30, What the Oath Rod Can Do, 672]

    

4. Romanda: Romanda was infuriated by Egwene's revelation about the Kin, the Oath Rod, and her idea that Aes Sedai nearing the lifespan limit imposed by the OR could unswear the Oaths and retire into the Kin:
   

   "When I was a little girl, I dreamed of becoming Aes Sedai. From the day I reached the White Tower, I tried to live as an Aes Sedai. I have lived as Aes Sedai, and I will die as Aes Sedai. This cannot be allowed!"
   [COT: 30, What the Oath Rod Can Do, 668]

    

5. Faolain: Faolain going MIA, together with her Red sympathies and her general bad attitude, certainly throws some suspicion her way. On the other hand, her absence could also add to the case for Lelaine. Faolain was supposed to be watching the Sitter; her disappearance could indicate that Lelaine got her out of the way (or, alternately, that she was collaborating with Lelaine).

    

6. Theodrin: Not much of a case for her, other than Egwene musing that she might be getting intentionally careless about watching Romanda (but then, who wouldn't hate that job?), and Rich Boyé's conviction that Theodrin is way too nice to be a good guy. But it is a possibility.

    

7. Delana/Halima: The strongest evidence in her favor is that Halima murdered Kairen Stang the night before the plan was to go into effect, thus eliminating one of the strongest cuendillar-makers in the Rebel camp and forcing Egwene to fall back on a half-trained novice as backup. (It evidently didn't occur to her that Egwene would decide to take over from Bode, though, since about the only good thing about Eg's capture is that it takes her out from under Halima's eye.)

   Halima would definitely have wanted the plan to fail and keep the two factions deadlocked, but in a strange way that counts as evidence against her - the capture of any Rebel AS, especially two who know how to Travel and make cuendillar, gives a distinct advantage to the Tower faction. Halima's interest is in keeping the conflict at a stalemate. Wouldn't a more efficient way of thwarting the plan while still giving no advantage to either side be to kill Leane and Bode, too? Why only Kairen?

    

8. Random Sitter: As noted, all the Sitters knew about the plan, and any one of them could have decided to turn traitor, though it's true that most of them would not have made a move without Lelaine or Romanda's say-so.