Which reminds me that it was only a few weeks ago when I first encountered the phrase "lost the plot," which I gather is quite common British slang. To "lose the plot" apparently means something like "lose the thread" or "lose perspective" or "stop making sense" or "get things in a mess" or (in the American idiomatic sense) "lose it." As in "Tories continue to lose the plot" or "[this was] before [the band] began to lose the plot" or "if you lose your temper ... you are certain to lose the plot." (Taken from assorted snippets of web pages that turned up in a Google search.)
Quasi-definitions thanks to Andrew Watt's 2002 post on xml-dev.