"To lose the plot" means to become confused, disorganized, or unable to think clearly, usually due to being overwhelmed, stressed, or distracted. Full definition
His stats are incredible when you consider some of the rubbish he's played with, and working under a manager who has completely lost the plot in every possible department of football management. (justarsenal.com)
Let me know if you think this tights - with - sandals thing is something you'd try, or if I've totally lost the plot. (thesequinist.com)
While sharing their perceptions of what actually happens, they highlight the danger of losing the plot during the action. (tes.com)