At Ben's graduation party, a family friend, Mr. McGuire, takes him aside and offers him a friendly piece of advice. What follows is one of the most quoted exchanges of dialogue in movies:

MR. MCGUIRE: I want to say one word to you. Just one word.

BENJAMIN: Yes, sir.

MR. MCGUIRE: Are you listening?

BENJAMIN: Yes, I am.

MR. MCGUIRE: Plastics.

What's wrong with plastics? From Benjamin's perspective, they're unnatural, artificial—well, technically, from anyone's perspective, plastics are artificial. You don't pick plastics off a plastics tree or dig them up in a plastics mine: you manufacture them. They're synthetic. To Benjamin's ears, the prospect of a future in plastics sounds artificial, boring, and soulless.

Ben wants his future to be "different"—and having a career in plastics, wouldn't, in his view, be different enough. There probably was a huge future in plastics at the time, but Ben's become disaffected with wealth and the pursuit of wealth. For Ben, plastics represented everything he despised and feared. In the 60s, the word "plastic" had come to mean anything or anyone cheap, superficial and artificial.

His relationship with Mrs. Robinson was plastic, in the sense of being phony and without feeling. Mr. McGuire's advice to Ben was representative of how clueless his generation was about what the younger generation found meaningful.

For an opposing viewpoint to Ben's take on plastics (can't live without 'em) check out this