After curing the Genophage, I got a message from the Salarian councilor. He said something about suspecting Udina of treason. Since I usually head back to the Citadel after missions, I went there to see about this next, but I ran into complications, as I somewhat expected. I was unable to contact Citadel Control for docking, but I did manage to get a message from Thane. He informed me that Cerberus had attacked the Citadel! I deployed my team to C-Sec HQ and worked with Bailey to regain control of things. When I found the Salarian Councilor, he was under attack by an assassin; a formidable seeming character I later learned was named Kai Leng. He would have surely succeeded in killing the defenseless Councilor, but Thane suddenly jumped in to save the day! The fight between these two assassins was fierce, but even though Thane was able to keep Kai Leng busy long enough that he couldn't kill the councilor, Thane was stabbed in the chest by Leng's sword. Kai Leng fled the scene, and the Salarian Councilor gave me proof that Udina was a traitor. I had no time to tend to Thane; I had to chase Leng to keep him from killing the other councilors. After a brief chase across the Presidium and up an elevator shaft I beat Leng to where the other councilors, including Udina, were being escorted by Ashley. When I confronted them and told them of Udina's treachery, Ashley was hesitant to trust me at first, but only for a brief moment. Without any extra persuasion on my part she turned on Udina who drew a gun on the other Councilor at which point I shot him dead. Leng never showed up again. Apparently he decided to flee the station, his mission failed.
My mission accomplished, I immediately went to visit Thane in the hospital. His son, Kolyat, was there to give him a transfusion, but it wasn't enough. Thane said his final prayers with Kolyat and I, and died. After this it was apparent the Citadel, formerly untouched by the Reaper war, had started gearing up for the war. As I looked around I met with a few more familiar faces. First, with the help of a C-Sec officer, I started tracking some old Batarian diplomatic codes someone had been using to commit acts of terror against humans on the Citadel. When I found the last one I was met with a gun to my back... being held by Balak, the Batarian terrorist who tried to destroy Tera Nova with an asteroid. He had apparently become the highest ranking Batarian left after their home systems had been taken by the Reapers. Much as we would have liked to kill each other, I persuaded him to work together with me, and he begrudgingly agreed and ran off, giving me control of the Batarian's remaining fleet (a rather sizable war asset actually).
Sadly, I had then found out that Kelly Chambers was killed by Cerberus while they were assaulting the Citadel. But exploring the area further I ran into an old 'friend' again: Conrad Verner. In a hilarious twist, he finally proved useful for something, and gave me his aid in building the Crucible, even though he was still a bit of a nuisance in some ways...
Next I was asked to investigate the Volus Ambassador on the Citadel: Din Korlack. The Turian Primarch had suspected him of being a Cerberus informant. When I found his office, I learned he was indeed an informant back when I was fighting the collectors, but he was rather unsuccessfully trying to break ties with Cerberus, due to their recent atrocities. Ceberus hired some mercenary assassins to kill Din, but I arrived on the scene in time to see one of them, my old friend Zaeed, kill the others. Apparently he had his own grudge against Cerberus and was taking jobs specifically against them, but got tricked into doing this one for them. When he found out that Cerberus was behind this job he joined me in persuading Din to not only inform on Cerberus's plans to attack a Turian colony, but also to grant me support from the Volus fleet. Zaeed himself also agreed to help in an advisory position for my war effort.
Back on the Normandy I had some more personal chats with several of my crew members: Traynor, my tech specialist, Diana Allers, the news reporter I have tagging along, and James. Though I seem to have had some options for romance here, I didn't pursue any of them, since I am still after either Garrus or Liara.
I have a lot of new tasks on my to do list now, and I am not sure which one to go for next...
By the way, Ashley joined my crew upon leaving the Citadel.