Green’s Dictionary of Slang

1. to lose one’s way in a situation, to miss the meaning or point.

1996		K. Lette Mad Cows 281: Like I said, you’ve lost the plot, Gillian.
2004		P. Howard PS, I Scored the Bridesmaids 15: I’m [...] thinking the goy has seriously lost the plot this time.
2005		B. Hare Urban Grimshaw 63: Greta [...] had lost the plot and started drinking heavily and taking drugs.
2007		P. O’Keeffe Hidden Soldier 136: [W]e were fighting mad now [...] I had completely lost the plot.
2010		D. Mason Marching With the Devil 347: That was the day Big Bird lost the plot. He stood there, yelling, screaming and visibly trembling.
2011		IOL News 5 Dec. 🌐The AIDS Gestapo has lost the plot.
2012		D. O’Donnell Locked Ward (2013) 318: He had gone apeshit [...] lost the plot and started to throw the furniture around.

2. to become incoherent or incapacitated through drugs or drink.

2000		Indep. Rev. 1 Mar. 7: Everyone lost the plot and collapsed on the floor.
2010		L. Redhead Thrill City [ebook] [She] got into ice. Lost a ton of weight and now she’s losing the plot.

3. to suffer a mental breakdown; to lose control of one’s emotions.

2001		A. Parkinson Cutting It Fine 236: There was a pastry chef, really under pressure, and one day he vanished from the kitchen. Someone went to the car park and found him lying on the ground, waving his arms and legs like a dying ant — he’d completely lost the plot.
2017		L. McInerney Blood Miracles : Dan lost the plot with him, handed him his arse.

what is happening?

1921

Jerry on the Job [comic strip] Hey – what’s the plot?? Where d’ya get that stuff?