I just wrote a (negative) review of the STTNG episode, IMAGINARY FRIEND, which is a very similar story.
I compared it to this LIS episode, which is EVERYTHING the STTNG episode isn't.
This story, of a lonely Penny, and her 'imaginary' friend, is so well-acted, by Angela Cartwright and, the handling - and this episode's ending STILL tug at my heart.
I don't want to write a synopsis, I just want to say -as others have, that, in it's first season, LIS, was a truly well done show, with much potential, and, it's on display brilliantly here.
If you want to see the a very similar idea done TOTALLY WRONG, check out the STTNG episode, IMAGINARY FRIEND.
Heck, I'm getting misty just writing this. 😌 (that's a good thing, for me).