How to cull an object that goes behind another object, with invisible mesh

I'm doing at. I want an object with a mesh [a], to move behind another object, which is invisible [b], and get culled as if [b] was not invisible.

I want to make my invisible object, an object I can put in place of a real wall, from the cameras image; but I don't want it to be actually visible.

That way I can make ar objects go behind windows, walls, doors and look nicer.

I have no idea how to achieve this. Help gladly received.