Warning: Wikipedia contains spoilers.
A political association develops a language, Babel 17, that can be used as a weapon of war. Simply learning it turns one into an unconscious traitor to one's own political association. This fact is discovered by a beautiful starship captain, who is very good at learning new languages. Usually she employs this skill to trade with aliens and get rich. This time, however, she is recruited by her government to go on a mission to discover how traitors are infiltrating and sabotaging strategic sites. She finds herself becoming a traitor as she learns the language. She is rescued by her dedicated crew, figures out the danger, and neutralizes its effects.
The barroom scene is identified by the book's fans as a classic and effective use of drama.