"The Grand Inquisitor" is a story embeded within THE BROTHERS KARAMAZOV. However, the story is "self-contained." Nothing is lost by reading it as a complete short story. In my opinion, "The Grand Inquisitor" is the greatest single story ever written. It's an interesting look at humanity, spirituality, and the church. If you are only going to read one more thing before you die, you must read "The Grand Inquisitor."
*This particular edition, however, seems to have some typos that were not caught by the proof-reader, but they're nothing very serious.