lose-the-plot

Verb

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

Used other than as an idiom: see lose,‎ plot.
(idiomatic) To cease to behave in a consistent and/or rational manner.
(idiomatic) To lose sight of an important objective or principle; to act contrarily to one's own interests through concentrating on relatively unimportant matters.

English Wiktionary. Available under CC-BY-SA license.