Lost the plot

What is the origin of the term "lost the plot"?

The metaphor is that as either a reader or a moviegoer you have lost track of the plot of the drama, and are saying "Hold on - was it the blonde who poisoned the Colonel? In which case, why was the butler lurking around on the landing? I thought he was in the pay of the guy with dark glasses? Oh, someone's stabbed him already? Then why the cigar stub on the dressing table? I don't know what's going on....".

I don't know when the phrase became commonplace, though. My feeling is that it hasn't been around for more than 20 years or so, at least in London where I live, but I could easily be wrong. (VSD)