When a brilliant but unruly CIA agent is unexpectedly assigned to find a missing Chinese dissident, he soon realises all is not what it seems. Not knowing who to trust, his loyalties to friends, family and country are tested to the limit.

The good news is it sounds like a movie.

The bad news is it sounds like many movies. It's just too generic. Give us some details. We don't know much about the CIA agent, the dissident, or what actually happens to make him realize everything is now what it seems.

As a general rule you should never have realize in a logline. It's an internal process. You need to tell us what happens. What event causes him to come to this realization.

Why is he unexpectedly assigned? Isn't that his job? Isn't this what he normally does if he's a CIA agent?

It's like Jason Bourne but without the cool hook.

Why is the assignment unexpected? Is that not his department? Is he a file clerk, or not a field agent?

Is the goal still to find the missing Chinese dissident? Or once he uncovers a conspiracy, does his goal change?

This is pretty good -- but 'his trust and loyalty to his friends, family and country will be tested to the limit' means you're left asking 'Why?'. I don't think you have to give the whole game away, just maybe be more specific in regards to the 'why'. The 'hook' here, in my mind, would be the fact that his friends and family are somehow involved -- I think the logline could make more of this:

'The CIA?s most brilliant but unruly agent will do anything to uncover the truth after discovering his friends, family, and the CIA itself are involved in the disappearance of a Chinese dissident.'

You could even be more specific as to WHO -- as in, his 'best friend' (?), his 'Brother' (?), his 'Wife' etc etc..

Anyway -- best of luck with it -- certainly has potential for a cool spy thriller...