You can add a modifier to the plane to make it point at the camera. Select the plane, go into the Modifiers tab, and choose Behaviors > Point At. Enter the name of the camera into the field. This will ensure that the plane always points at the camera, but it doesn't ensure that the plane is always in view. If you want you can also set up a modifier for the camera to make it always point at the plane. Or if it makes sense for your scene you can just group the plane and camera and move them together as a unit.