Kolyat has received word that, at the request of Donnel Udina, the Citadel Council will hold a critical vote in 24 hours that will abandon the hanar homeworld to the Reapers in the name of protecting other planets. He needs you to get to Udina, one way or the other. He brings in Kasumi (if alive) to help nab evidence from C-SEC headquarters he could use to blackmail Udina. You get in to C-SEC HQ, and finds the evidence being destroyed by a Cerberus double agent. Extracting the information from him, you discover that Udina is indoctrinated and will soon launch a coup d'état to overthrow the Citadel Council. You and Kolyat must now stop him through any means possible. Thane (if alive) is brought on board to assassinate Udina. Thane is weak from his disease, but is on heavy painkillers and determined to complete his last mission.
You, Thane, and Kasumi take on heavy security measures on their way to the Council Chamber in the Citadel Tower. Thane and Kasumi's presence helps you get past various puzzle obstacles. A short but brutal fight with Cerberus conspirators ensues when you secure a vantage point to take the target down. When Udina makes his move, you are there to reveal his crimes in front of the Council. Udina reacts by signaling an army of hidden Cerberus forces. In the climactic showdown, Thane uses the last of his strength to take out Udina. You give Thane the last rites he is too weak to say for himself, and Thane dies, his species' debt to the hanar repaid.
The grateful Council is now completely behind Shepard and give him their unconditional support.