Lose the plot
Definition 1 of 2
Pro Tip 1/3Emotional Overload
Used when someone overreacts or gets too emotional about a situation. 
When he was fired, Dan lost the plot and shouted at his boss.
Pro Tip 2/3Colloquial Usage
Common in spoken English to describe extreme reactions, but not suitable for formal writing. 
She lost the plot when she saw the mess after the party.
Pro Tip 3/3Not Literal
It doesn't mean someone actually loses something; it's a metaphorical expression. 
My dad completely lost the plot during the football match.