what's the big idea
What is the purpose; what do you have in mind; why did you do that; what are you doing; how dare you. — Often used to question someone or something that is not welcome.
The Smith family painted their house red, white, and blue. What's the big idea?
What's the idea of coming in here after I told you not to?
I heard you are spreading false rumors about me, what's the big idea?