Capacitors in series

I read about an example in which you had two metal plates, and in between them one third of the distance from the top plate downwards (towards the bottom plate) was made up of κ, and the rest was air. THe problem proceeded to calculate the capacitance of them in series.

I don't get how the top layer acts as a capacitor though. There's no metal plate? Can someone please help me understand what's going on?