At 3:53 when attempting to move the ground plane I can not move it through the camera view. The back of the plane goes down while the front remains fixated.
Kenan is not moving the ground plane (you want the ground plane to stay on the 3D 'ground') there, he is moving the Camera up (G, Z with the Camera selected) and rotating it a bit (with R) to fit the footage.
When the back of the plane goes down and the front stays, then you are rotating it (on the X-axis if your scene is set up the same as in the tutorial).