A line #d# is always contained in a plane.

Either #d# is contained in a plane parrallel to the plane #alpha#, and then #d nn alpha = O/#.

Or #d# is contained in a plan #beta# that is not parallel to #alpha#, in that case #beta nn alpha = gamma# where #gamma# is a line, and #gamma nn d != O/#, which means the 2 lines intercept in 1 point, and this point is included in the plane #alpha#. I hope you understood, don't hesitate to ask.