Pen Pals is one of those 'Good for season 2' episodes.
But it did establish the ridiculous precedent of the prime directive meaning 'We can't interfere with the deaths of millions', a precedent which created a good part of the series' biggest facepalm moments. Dear Doctor is this episode's fault. For a society based on science and rational thinking, they were sure superstitious about the 'Plan of the universe'. As if they, by their decisions, are big enough to single-handedly defy the universe's great plan.