lose the plot

verb2

1. •
   (UK, informal) 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
2. •
   (UK, informal) To lose sight of an important objective or principle; to act contrarily to one's own interests through concentrating on relatively unimportant matters.