lose the plot
1 lose your ability to understand what is happening; lose touch with reality.
2 become uncontrollably angry; lose it. informal
❶ 2013Daily Telegraph As voters switch off from the David v Goliath contests favoured by the leader, both foes and friends are starting to ask whether Mr Miliband has lost the plot.
❷ 2002Glamour On holiday in Spain I lost the plot because I thought my boyfriend was looking at a girl in the distance. I pushed him into a swimming pool.