Lose the plot means: You lose control of your mind and become completely insane. Making stupid or irrational choices. Most cases last for a short time, but some can become permanent and will need to be inhabited in mental hospitals together with men wearing white coats. (in Community Dictionary, added by Eugenia Márquez)
What else does Lose the plot mean?
- To be unable to understand or cope with the world around you. (in Community Dictionary, added by Israel Marshall)
- Get angry, lose control and get really mad. It is derogatory because it implies that the reaction was not reasonable in light of the circumstances. (in Community Dictionary, added by Camilla Perez)