From "Modern Family". Statement intended to mock and express bewilderment at someone else's actions. Similar sentiment to "what the hell are you doing!?"
In Modern Family S02.E01, Phil and Claire's family exit the old family station wagon. It begins rolling backwards towards the cliff. Phil's reaction, rather than to try to get into the car and hit the brake, is to leap on the hood and grab on, spread-eagled. Claire looks at him with a baffled expression and screams, "What's the plan, Phil?!"