Lose the plot (primarily heard in the UK)
colloquial phrase

1. To act in a disorganized, chaotic, or irrational manner.
"Bob had one too many drinks last night and just LOST THE PLOT.

2. To lose focus on one's primary objective, principle, or task.
"We agreed to develop the product together, but after a promising start, our business partners completely LOST THE PLOT." 

The Farlex Dictionary

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ORIGIN

This metaphor means that as either a reader, a theatre goer or moviegoer one has lost track of the plot of the drama.

There is a dubious 17th-century citation, with the next not coming until a 1984 quote from The Times, presumably about a fashion show:

“Arabella Pollen showed sharp linens, LOST THE PLOT in a sarong skirt and brought out curvaceous racing silk and a show-stopping bow-legged Willie Carson.”

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SYNONYMS

The Oxford English Dictionary defines “lose the plot” as “to lose one’s ability to understand or cope with events; to lose one’s touch; to go off the rails.” You can also say: to be at sea, to be not with it, to be out of touch with reality