Shepard is able to take control of the Normandy again and stop Saren. Somehow Udina manages to survive the initial attack on the Citadel when Sovereign and Saren launch their assault.

Was Udina a traitor?

Shepard must pursue him to reach the rest of the Council after learning that Udina is a traitor. He's planning to sell the Council out to Cerberus, and he's currently leading the rest of them to their death.

What happens to Anderson if you choose Udina?

The unfortunate thing is that no matter what the player chooses, by Mass Effect 3, Udina will have been given the role of Councilor and Anderson will have been removed from the board. While this choice is made up to be a big one at the end of Mass Effect 1, its widespread ramifications aren't particularly deep.