Lose the plot is an informal British English idiom. It means to have no idea what is going on, either in a sequence of events or with reality in general. This idiom is used in all English-speaking countries.
Related idioms include being lost in a conversation or situation. This is not the same as when you are actually lost, as in not knowing which direction to move.
Another related idiom is lose one’s marbles, which means to be losing one’s sanity or connection with reality.
Examples
Despite all such efforts, the district began to lose the plot when it dropped its Minchinna Sanchar programme, which was introduced two years ago, says an educationist. [Times of India]
Has the Malaysian government so clearly lost the plot that even outside observers would recognize that the trial was blatantly political from day one? [CNN]
Offaly’s Shane Lowry was forced to display his skill with a wedge after losing the plot and breaking his putter on the 12th hole at the Dubai Duty Free Irish Open at Royal County Down. [The Irish Independent]
Just as André loses the plot, so do we, until we are thoroughly disorientated. [Financial Times]
I could not get a movie, a play, a TV show, or even an ad campaign to save my life. I felt so lost and derailed, because all I had ever known was working. [New York Magazine]
My then-husband thought I was crazy, my friends thought I was crazy, my colleagues thought I had lost my marbles. [Huffington Post]