Newbie question about Planes and intersection

I can't seem to get intersection with a plane to work correctly. Right now I'm suspecting it's because I'm doing something wrong.

I want to have a plane facing upwards (Y-direction) at a certain height. I then want to find out where my predefined rays intersect with this plane.

Plane p = new Plane( Vector3f( 0, 1, 0 ), upperBound ) //Where upperBound is my Y-height 
r.intersectsWherePlane( p, loc2 );  // Where r is a ray that I know has Y-coordinates above and below upperBound

But I only get nullvalues in loc2. So my question is, where do I go wrong? What should I pass to the Plane constructor to get a upward facing plane at a certain height?

I just implimented this in my program last night…

This thread helped me a lot: