lost the plot

Verb

lose the plot (third-person singular simple present loses the plot, present participle losing the plot, simple past and past participle lost the plot)

1. (UK, colloquial, idiomatic) To cease to behave in a consistent or rational manner.
   • I lost the plot for a while then. And I lost the subplot, the script, the soundtrack, the intermission, my popcorn, the credits and the exit sign.1995, Nick Hornby, High Fidelity, London: Victor Gollancz, →ISBN, page 25
2. (UK, colloquial, idiomatic) To lose sight of an important objective or principle; to act contrarily to one's own interests through concentrating on relatively unimportant matters.
   • But while there remains a considerable degree of consensus that the consequence of apparently losing the plot sometime between 1914 and 1918 was the cultural and economic malaise of the 1920s and 1930s, there are still some who look back on the interwar years less with criticism than with nostalgia.1995, John Frank Williams, The Quarantined Culture: Australian Reactions to Modernism, 1913–1939, page 219
3. Used other than figuratively or idiomatically: see lose, plot.
   • Readers enter the storyworlds of novels and then follow the logic of the events that occur in them primarily by attempting to reconstruct the fictional minds of the characters in that storyworld. Otherwise, readers lose the plot.2005 Winter, Alan Palmer, “Intermental Thought in the Novel: The Middlemarch Mind.”, in Style, volume 39, number 4, page 427