Projection matrix + world plane ~> Homography from image plane to world plane

Question

I think I have wires on this, it should be pretty easy.

I have a projection matrix from world coordinates to image coordinates (4D homogeneous to 3D homgeneous), and therefore I also have an inverse projection matrix from image coordinates to world "rays".

I want to project image points back onto a plane in the world (which, of course, is given as a 4D homogeneous vector). The necessary homography must be uniquely identified, but I cannot figure out how to calculate it.

Of course, I could also intersect the back projections of the rays with the world plane, but that doesn't seem very good, knowing that SHOULD be a homography doing it for me.

Thanks in advance Ben