It's about a bunch of aliens that come down to earth and help all the humans with their super alien technology but are actually only doing that so they can eat us later. Some humans decipher the title of the book that is being handed out to all the alien ambassadors and it's called "To Serve Man". At first they think it means serve as in servant but it turns out it means serve as in prepare as food.
I love how SOMEHOW they manage to translate the name of the book into a double-entendre without knowing what it originally meant? How the hell does that happen, anyway? Does the same double-entendre exist in the alien language too?
Seriously, I love the old The Twilight Zone, but the linguistics in that episode were about on par with the worst Dan Brown could ever throw at you.
I mean, think about it. You've got some alien phrase that means "To serve man [to someone as food]". How in the name of God do you get "to serve man" in English out of that, yet without knowing what the original text actually meant? The entire episode managed to treat the alien language as some kind of substitution-cipher of English, like it was the exact same language but with different letters or something.