Plane collision question

I'm programming a 3D editor and I didn't find a answee to this quesion in my maths book so.. anyone help me please.
I got a flat 3D plane, for example:

A = 0, 0, 0
B = 0, 10, 0
C = 10, 10, 0
D = 10, 0, 0

Now this plane is in 3D space, therefore translated, rotated and projected by the camera matrix onto the screen. The mouse pointer which is of course in screen coordinates should now be able to point on this plane, you know you move the mouse over the 3D plane and the selected area lights up. So what I need is: where does to mouse 'laser' hit the plane? Any ideas how to solve this?

Here's an illustration to the example mentioned above. As you can see the mouse clicks on 196, 132 and hits the plane at 6, 7 or something (Z is ignored because it's always 0).

Rotation, Translation and Projection matrix are known D3DMATRIX values.