How do I make an object which hides 3d objects behind it, but shows my video background? I am currently interested in tracking 3d objects into my videos. I am not sure how to create what looks like a hole in the ground. If I used a sqaure with no top face, the outer sides of the cube would still appear over the video, I need only the inside of the cube to be seen.

Is it possible to make a sort of plane object that hides the 3d objects behind it, but still shows my video background?

-Owen
As Mechanic says, what you ask is not clear, please show some screenshots. Do you want to create an alpha transparency wherever you want on your 3D object? The Holdout shader does that. Don't forget to activate the Properties panel > Render > Film > Transparent option.