Alias in as small a nutshell as I can put it...
(Spoilers)
Jennifer Garner is Sydney. She works for the CIA, or so she thought. She ends up being contacts by the REAL CIA and they tell her who she's working for...I think, or else it was her dad.
Anyways, she finds out she's actually working for SD6, a bad group of men hell bent on doing bad things. The head of SD6 is Sloan, and he wants to find these Rembaldi artifacts that will unlock something huge, we don't really know yet, but it might be time travel related.
So now that Sydney knows she's working for SD6, she becomes a double-agent, working FOR the CIA to infiltrate SD6.
So that was basically the first season, her doing things for the CIA to help them keep tabs on SD6.
However, this season ended up doing all kinds of things, like introducing her monther, who killed a ton of CIA agents before Syd was born, and was thought to have been killed.
Then SD6 was finally brought down by the CIA and Sydney, now Sloan is on the loose.
However, at the end of last season, Sydney gets in a fight with her roommate, who really isnt' her roommate, and wakes up TWO YEARS after the fight in China, without any recollection of what happened.
There is a lot that happened that I didn't mention, but you could read some synopsis of past episodes on their website.
Mike