Babel-17

Note that wikipedia has spoilers[?].

A political association develops a language Babel 17 that is a weapon of war. Simply learning it turns one into an unconscious traitor to one's political association. This is discovered by a beautiful starship captain, who is (what else?) a super-whiz at learning new languages. Usually she employs this skill to trade with aliens, and get rich. This time, she is recruited by her government on a mission of derring-do: How are traitors infiltrating our government? She finds herself becoming a traitor as she learns the language. She is rescued by her dedicated crew (Captain, why are you planting bombs?), figures out the danger, and neutralizes the effects.

The bar scene alone is worth reading the book. The creepiness of finding oneself (as the captain) doing things of which one consciously disapproves, is also marvelous.